How do you explain this one?

The monthly Market Share survey from Net Applications, which had reported steady increases for the Mac OS in the previous two months and sharp growth for the iPhone — including a 57% surge in August — showed a very different picture in the October report issued overnight Saturday.

In October, iPhone growth slowed to 3.12% (down from 6.67% in September) and the Mac’s share actually fell, down 0.24% from September. (Linux was particularly hard hit, down nearly 22% for the month; see charts below.)

Windows, meanwhile, got a little bump — up 0.19% — thanks to a healthy 5.24% jump in Vista’s share.

How did this happen?

The first thing to be said about these results is that Net Applications’ “market share” report doesn’t actually measure share of market as a percentage of revenue or unit sales. That’s the business Gartner and IDC are in. And in Gartner and IDC’s latest reports, Apple’s (AAPL) share of the U.S. market had grown to 9.5% and 9.1% respectively, largely at the expense of the HP (HPQ) and Dell (DELL) (see here).

What Net Applications does measure — based on browser data from some 160 million visits to websites operated by Net Applications’ clients — is the extent to which users of each operating system are hanging out on the Internet.

In other words, what the Web metrics firm’s latest data show is that in October, Windows users — and Vista users in particular — were coming online at a faster rate than Mac users. This despite the fact that Mac sales grew 21% worldwide last quarter according to Apple, and roughly 30% in the U.S. according to Gartner and IDC.

So what was it about October that drew Vista users to the Web in greater proportion than Mac users? Could it have something to do with Microsoft’s (MSFT) $300 million ad campaign for Windows? Was it PC users obsessively tracking poll numbers in the U.S. presidential race? Could it be related somehow to the meltdown of the global financial markets?

While you ponder that, here are summaries of the latest Net Applications reports, broken down first by OS and then by different versions of those operating systems. To see the full results, click here.

The presence on the Internet of both the Mac and the iPhone grew smartly this summer, registering record numbers for each operating system in the Net Applications survey issued overnight Wednesday.

The Mac’s share of Web hits grew 5.34% to a record 8.28% last month, according to the Web metrics firm’s September survey. The iPhone’s share, having surged 57% — from 0.19% to 0.30% — in August (see here), grew another 6.67% to hit 0.32% in September.

These monthly surveys are conducted by sampling browser data from some 160 million visits to websites operated by Net Applications’ clients. Although the company describes the results as “market shares,” Net Applications does not actually measure share of market in the traditional sense of sales revenue or unit sales. It does, however, provide a consistent methodology by which to measure operating system trends.

By this yardstick, Microsoft’s (MSFT) Windows, while still dominant with a 90.23% share, has been slowly losing ground. Apple’s (AAPL) operating systems, by contrast, have been gaining share at a fairly brisk pace, albeit from a much smaller base.

To see Net Applications’ Oct. 1 report, click here. The results are summarized in the table below.

To get a feel for how Internet use compares with unit sales, check out the chart that Alexis W. Cabot has posted on The Mac Observer’s Apple Finance Board using the latest numbers from Gartner and Net Applications:

Source: Net Applications

The iPhone’s growing presence on the Web, having leveled off before the introduction of the iPhone 3G, surged in the month and a half since, according to preliminary data released Sunday by Net Applications, an Aliso Viejo, CA-based Web service company.

The percentage of Web hits coming from iPhones passed 0.2% in June and then dipped in the weeks that followed. But it peaked on August 23rd at a record high 0.48%, according to the new data, before drifting back last week.

Net Applications’ brief report, issued in advance of its August survey of operating system market share data, offered no explanation for last week’s fall-off, but it did attribute the jump in July and August to the flood of iPhones 3Gs sold by Apple (AAPL) and its partners since the device’s July 11 launch.

[In its August survey, released early Monday, Net Applications reported that the iPhone's share of global Web usage increased 58% in the course of the month, climbing from 0.19% in July to 0.30% in August. In other words, one out of every 333 Web hits in August came from an iPhone. See here.]

The iPhone has cast an oversize shadow on the Internet from the moment the original model was introduced in late June 2007. By the time Net Applications issued its July 2007 survey, the iPhone already represented 0.04% of the visits to websites operated by the firm’s clients. That’s more than double what one would expect, given that there were about 1.4 billion computers connected to the Internet at that point, according to Internet World Stats, and only 270,000 or so were iPhones.

You can see the latest data at the New Applications website here. The company’s surveys are based on data collected from the browsers of visitors — some 160 million per month — to its customers websites.

In the August survey issued Monday, the iPhone OS, with a 0.30% share, was the fourth-most popular operating system on the Web after Windows (90.69%), Mac OS (7.84%) and Linux (0.92%).

Microsoft (MSFT) Windows continued its downward drift and Apple’s (AAPL) Mac OS X inched up to a record 7.95% in the market share survey issued Tuesday by Aliso Viejo, Calif.-based Net Applications.

The biggest gain, however, was recorded by the open-source operating system Linux, which jumped more than 16% in June — albeit from a small base — to hit 0.79%.

The iPhone held steady at 0.16%, reflecting a leveling off of what had been double-digit growth as buyers waited for the new iPhone 3G, which goes on sale next week. In a separate survey issued Monday, RBC Capital reported “unprecedented pent-up demand” for the new model. Data taken from 3,600 members in RBC’s Technology Adoption Panel in early June showed that 56% of those planning to buy a smartphone in the next 90 days planned to buy an iPhone — up from 35% in March and more than double the interest in any of the other brands surveyed. See here for more detail.

The monthly Net Applications survey is conducted by sampling browser data from some 160 million visits to websites operated by the firm’s clients. Although it describes the results as “market share,” Net Applications does not actually measure share of market in the traditional sense by revenue or unit sales. It does, however, provide a consistent methodology by which to measure operating system trends.

To see their July 1 report, click here. The results are summarized in the table below.

Drilling deeper into the numbers, ArsTechnica’s Charles Jade notes that the numbers for Intel Macs grew by a quarter of a percent to 5.26 percent, while PPC Mac’s declined to 2.7 percent. In other words, Intel Macs increased at twice the pace of decline for PPCs. “The rapid decline of PPC Macs coupled with sharp gains for Intel Macs no doubt factored into the decision to make Snow Leopard Intel only,” Jade speculates. His chart below:

UPDATE: Net Applications’ model must be more dynamic than we knew. At sunrise in New York on Monday, Mac’s June share was 7.95%. By 8:30 a.m. CT, when Jade posted his report, it had risen to 7.96%. By 3:00 a.m. ET Tuesday it had dropped to 7.94%. And these are last month’s numbers!

After drifting inexplicably in February, March and April — actually losing market share in two out of three months just when Macintosh sales seemed to be on fire — Mac OS X recovered smartly in the Net Applications survey issued overnight Sunday.

Apple’s (AAPL) share of the operating system market grew 5.69% in May to hit a record 7.80%, while Windows in all its flavors dropped half a point to 91.17%. That’s a record low for Microsoft (MSFT), which nonetheless still runs on 9 out of 10 computers on the Internet, as Net Applications measures it (more on its methodology below).

The iPhone’s OS market share, whch Net Applications measures separately from OS X, has temporarily leveled off, according to the report, reflecting the shortage of product as Apple cleared inventory in May and customers held off purchases in anticipation of the new 3G model. In an IDC report issued Friday, the iPhone actually lost share in the smartphone market, falling from 26.7% in the last quarter of 2007 to 19.2% in the first quarter of 2008. RIM (RIMM), meanwhile, gained share in the same period, growing from 35.1% to 44.5% on the strength of new, consumer-oriented BlackBerries. (see here)

The discrepancy between IDC’s and NetApplication’s numbers can be explained to some extent by the nature of the two surveys. IDC’s quarterly reports are sales counts, based on surveys of retail outlets. Net Applications, by contrast, collects data from the browsers of visitors — some 160 million per month — to its customers websites. As such, its findings are probably better described as a snapshot of installed base taken from a less-than-random sample. But the results are useful for indicating trends, and tend to correspond well to domestic market share as measured by more traditional methods.

To see Net Application’s June 1 report, click here. The results are summarized in the table below:

Subscribers: to see the chart, click here.

On March 18, along with the latest version of iTunes and QuickTime, Apple slipped a copy of Safari 3.1 into the Software Update it sent to millions of Windows users — even though strictly speaking the first non-beta version of Safari for Windows was a new program and not an “update.”

Critics, among them longtime Apple supporters, excoriated the company for what was widely viewed as an uncharacteristic sleight of hand. They called it “disgraceful,” “malware” and a violation of the “trust relationship great companies have with their customers.” (See for example here)

What they didn’t call it was effective. But data released on Thursday by Net Applications show that the brief experiment worked rather well. During the month that it lasted, the percentage of Safari for Windows users among Net Applications’ clients, which had never climbed above .07%, grew three-fold, to .21%.

It might also have helped that the program was getting good reviews, although it’s not clear how many Microsoft (MSFT) Windows users would ever have tried Apple’s (AAPL) Web browser if it hadn’t been shoved in their face.

On April 18, Apple revised its Software Update protocol. New programs are now clearly marked as such and the box to accept them is unchecked by default.

After showing impressive gains in December and January, Mac OS X’s slice shrank a bit last month in the latest Net Applications survey of operating system market share. The iPhone’s share growth, meanwhile, continues to outpace every other category except “Other.”

As Net Applications measures it (more on its quirky methodology below), the Mac’s market share dipped to 7.46 percent, 1.45 percent off its January record high. Apple’s (AAPL) iPhone, meanwhile, hit 0.15 percent, a personal best, up 7.69 percent from January.

Microsoft (MSFT) Windows in all its flavors continues to dominate, with a 91.58 percent share, up a hair for the month but still off the 93.3 percent it held a year ago.

Net Applications’ monthly surveys sample data from visitors to some 40,000 websites operated by the firm’s clients. As such, the findings are a snapshot of installed base — with a bias toward machines that spend a lot of time on the Net — rather than a month-to-month measure of computer systems sold. The February results are summarized in the table below. The full report can be viewed here.

The news was almost lost yesterday in the Microhoo hoopla: even as Microsoft (MSFT) was trying to buy its way into some kind of parity with Google (GOOG) by gobbling up Yahoo (YHOO), a new survey showed that Apple (AAPL) had taken another little bite out of Redmond’s core business.

According to the latest market share data from Net Applications, Mac OS X’s slice of the computer operating system market grew 3.56% in January while Microsoft Windows’ dropped .36%.

As Net Applications measures it (more on its methodology below), the Mac’s market share stands at a record 7.57%, up 21.7% from Jan. 2007.

“Apple’s market share gains in December for the Mac and iPhone are impressive,” the report concludes. “However, for the last days of December, the numbers are nothing short of spectacular.”

The really good news for Apple, according to Net Applications, came in the last two days of the month, when Mac OS X hit 8.01%.

Windows is still dominates the desktop, of course, with a 91.46% share, but that’s down from 93.33% a year earlier.

Net Applications’ monthly surveys sample data from visitors to some 40,000 websites operated by the firm’s clients. As such, the findings are a snapshot of installed base rather than a month-to-month measure of computer systems sold. The January results are summarized in the table below. The full report can be viewed here.

Reflecting strong holiday sales of both MacBooks and iPhones, Apple’s (AAPL) market share grew sharply in December, as measured by a Net Applications survey released today.

The Mac hit a record 7.3% share, up from 6.8% last month. The iPhone also hit a new record, .12%, up from .09% in November. That suggests that better than 1 out of every 1,000 people on the Internet are browsing the Web using an iPhone.

Microsoft’s (MSFT) Windows still dominates, with a 91.8% share as measured by the Web metrics company. But it lost ground in December, as it has for seven of the past 11 months.

The Mac OS share, by contrast, grew 7.4% in the past month, nearly double November’s rate. The iPhone grew even more sharply, jumping 33% over November’s numbers. Only the Playstation (.02% share) grew faster, albeit from a much smaller base.

Net Applications’ monthly surveys do not measure market share in terms of computer systems sold. Rather, they sample data from visitors to some 40,000 websites operated by the firm’s clients. As such, the findings are probably better described as a snapshot of installed base taken from a less-than-random sample. But the results tend to correspond well to domestic market share as measured by more traditional market survey firms like IDC and Gartner. To see Net Application’s full report, click here.

The Linux operating system also showed strong growth (up better than 10% to hit a .63% share), as did “other,” a category that includes the iPod touch, Web TV and the Nintendo Wii.

The results are summarized in the table below:

After taking a brief October dip in advance of Leopard’s release, Apple’s (AAPL) share of the operating system market grew 3.34% in November to hit a record 6.81%, according to the results of a Net Applications survey issued today.

Microsoft’s (MSFT) Windows in its various flavors continues to dominate with a 92.42% share, as measured by the Web metrics firm. Among the operating systems gathered in the “other” category are Linux (.57%), Apple’s iPhone (.09%), Sony’s Playstation (.02%), SunOS (.01%) and Nintendo’s Wii (.01%).

Net Applications’ monthly surveys do not measure market share in terms of computer systems sold. Rather, they sample data from visitors to some 40,000 websites operated by their clients. As such, their findings are probably better described as a snapshot of installed base taken from a less than random sample. But they do reflect market share trends, and it’s always interesting to compare their results with those from firms like Gartner and IDC, which track quarterly shipments.

Net Applications’ October report, for example, showed Apple with a 6.61% market share. A couple weeks earlier, IDC had calculated Apple’s domestic market share in terms of units shipped in the 3rd quarter at 6.3% while Gartner’s estimate for the same period came in at 8.1%. (link)

Net Applications’ November results are summarized in the table below: