One out of ten people browsing the Web in December were using a version of Mac OS X, according to the monthly statistics from Web services company Net Applications.

Users of Mac OS X on Intel (7.19) and PowerPC (2.44) Macs combined for 9.63 percent; iPhone users added 0.44 percent and iPod Touch users another 0.08 percent.

The total of 10.15 percent marks the highest share of users ever for Mac OS X in the Net Applications stats.

A point of clarification: although Net Applications describes its data as “market share,” a more accurate description would be “user share,” as in the percentage of people on the Web using a particular operating system or Web browser.

The free monthly statistics derive from data the company gathers in the course of its business -- measuring Web traffic for its clients. It’s not a scientific process, but the sample is huge – 160 million Web site visits – so the data is useful in detecting trends.

Back in June I noted the Mac’s share had more than doubled from 4.26 percent in June 2006 to 7.83 percent in May 2008. With share now over 10 percent, the upward trend for Mac OS X appears to be accelerating.

A disclaimer on the Net Applications site warns “the December holiday season strongly favored residential over business usage. This in turn increases the relative usage share of Mac, Firefox, Safari and other products that have relatively high residential usage,” but the trend line for OS X had virtually the same trajectory in November (see chart).

The numbers jibe with market share reports on sales from such firms as IDC and Gartner, both of which reported Apple’s U.S. market share in the third quarter of 2008 at about 9.5 percent.

And while most of the gains in the Net Applications stats have come from people using Apple’s computers, one can’t dismiss the iPhone. In fact, one could argue the iPhone’s rise to 0.44 percent is more impressive than the Mac’s gains. We’re talking about a phone, after all.

Compare the iPhone’s number against other smartphone operating systems: Windows CE managed only 0.05 percent and the mighty BlackBerry a tiny 0.01 percent. The iPod Touch – not even a smart phone itself -- had more than both combined.

In the battle for dominance among mobile devices on the Web, Apple has the clear early lead.

Meanwhile, Microsoft’s Windows has continued its slow erosion in share, having slipped below 90 percent (hitting 89.62) for the first time in the November survey. The December stats put Windows share at 88.68 percent, nearly a full percentage point drop in one month.

Windows accounted for 95.25 percent of Web users in the June 2006 survey, so we’re looking at a loss of 6.57 percent compared to Mac OS X’s gain of 5.89 percent over the same period.

Of greater concern to Microsoft might be the relatively small adoption rate of Windows Vista. More than 65 percent of Windows users are still on XP, with only 21.12 percent on Vista. The next version of Windows, Windows 7, is expected in early 2010, so many users may bypass Vista altogether.

And if Microsoft chief Steve Ballmer wants yet something else to stew over, he can check the browser usage numbers. Internet Explorer, which had 84.11 percent share in June 2006, now has only 68.15 percent, a nearly 16 point fall; IE lost 1.62 percent just from November.

Safari has taken some share, having risen from 3.19 percent in June 2006 to 7.93 percent in December 2008, but Firefox has emerged as a significant threat. As of December Firefox has a share of 21.34 percent, nearly twice the 10.77 percent it had in June 2006.

Methinks the era of Microsoft hegemony is crumbling fast.