Microsoft's Windows resumed its usual losing form last month as the operating system's usage share dropped by about a third of a point even as the new Windows 7 posted a second straight month of impressive gains, according to web metrics firm Net Applications.
Although rival desktop operating systems - Mac and Linux - essentially remained flat, mobile OSes, including Google's Android and Apple's iPhone OS, took up the slack created by Windows' dip. Mobile operating systems, said Net Applications, now power 1.3% of all the hardware that surfs the internet.
Windows finished the year with a 92.2% share, down 0.3 of a percentage point. It was the eighth month in 2009 during which Windows lost share.
As it did in 2008, Windows' decline again accelerated in the second half of the year, when it lost 1.2 points of share. That compared to a drop of just 0.5 of a percentage point in the first six months of 2009. In 2008, Windows also lost more than twice as much share between July and December as it did in the preceding six months.
But the slip doesn't mean Windows is in any danger of losing its grip on the operating system market anytime soon: at the pace of the last 24 months, Windows would retain a majority share for another 27 years.
As in November, both Windows XP and Windows Vista lost share in December, while Windows 7 gained ground. Unlike in November, however, Windows 7 was unable to make up for the decline in Microsoft 's older operating systems.
Windows XP slid 1.3 percentage points in December, its second-largest one-month decline ever. (The record remains November, when XP lost 1.4 points.)
Vista, meanwhile, lost 0.7 of a percentage point, a single-month record, to end at 17.9%. December was the second month in a row that Vista lost share, and the third in the last four months, a trend that points to a permanent decline as users abandon it for Windows 7.
Still, the bulk of Microsoft's losses since the Windows 7 launch on October 22 have been from Windows XP; the eight-year-old OS has lost 2.7 points in the last two months, while Vista has lost only 1 point.
Microsoft's newest OS, on the other hand, boosted its share by 1.7 percentage points to end December with 5.7%, meaning that approximately 1 out of every 18 machines on the web ran Windows 7 last month. If it can keep up the pace of the last 60 days, Windows 7 will crack 7% this month, beating Vista to that number by six months.
Windows 7 also reached a milestone on January 1, 2010, when it posted an 8% share for the day. The previous one-day record of 7.6% had been set on December 27, 2009.
Apple's Mac OS X dipped for the second month in a row, finishing December with 5.1% after a decline of a statistically insignificant 0.01 of a percentage point. Most months, however, Mac OS X posts gains, not losses: December was only the fifth month of 2009 in which Apple's operating system lost share.
The winner, according to Net Applications: mobile operating systems, which accounted for 1.3% of all OSes powering devices that browsed the internet in December. Although their shares remained small - the largest was Java Platform, Micro Edition, with just 0.53%, followed by the iPhone OS with 0.44% - month-over-month increases were dramatic in some cases. Google's Android operating system, for example, increased its share by nearly 56% between November and December, while RIM and the iPhone boosted their shares by 22% and 20%, respectively.
Net Applications measures operating system usage by tracking the machines that surf to the 40,000 sites it monitors for clients, which results in a pool of about 160 million unique visitors each month. It then weights share by the estimated size of each country's Internet population.
December's operating system data can be found on Net Applications' site.