October Internet use: Vista up, Mac down by Philip Elmer-DeWitt @FortuneMagazine November 1, 2008, 2:09 PM EDT E-mail Tweet Facebook Google Plus Linkedin Share icons 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.