One in five Mac users has adopted OS X Mountain Lion, the upgrade launched five weeks ago, Web analytics company Net Applications said Saturday.

Meanwhile, Microsoft's Windows 7 passed the 11-year-old Windows XP for the first time last month, accounting for 46.6% of all Windows machines.

According to Net Applications, Mountain Lion, also known as OS X 10.8, averaged a 20% share of all Macs in August, an increase of 16 percentage points over the 4% it posted during July. Apple shipped Mountain Lion on July 25.

Mountain Lion's increase was sharper than that of its predecessor. In August 2011, OS X Lion averaged a 17% share of all Macs, a gain of 11.5 percentage points from the previous month. Apple launched Lion on July 20, 2011.

While others, including the Chitika online ad network, have also tracked Mountain Lion's uptake, Net Applications' numbers were nearly double Chitika's: Last week, the ad company said OS X 10.8 had garnered a 10% share of all Macs during the last week of August.

Most of Mountain Lion's gains came at the expense of Lion, whose share of all Macs plunged from 47% to 34%. OS X Snow Leopard lost just two percentage points in August, a loss consistent with earlier months, while Leopard, aka OS X 10.5, dropped by less than one point.

The quick rise of Mountain Lion wasn't surprising: Apple priced the upgrade at $19.99, a third less than Lion's price last year.

On the Windows side, Net Applications said Windows 7, which debuted in October 2009, finally became the most popular edition of Microsoft's operating system, beating Windows XP 46.6% to 46.3%.

Net Applications and Computerworld had predicted that Windows 7 would best XP in July, but the aged OS defied prognostications and hung onto the top spot one more month instead.

Net Applications' rival, Irish analytics company StatCounter, first pegged Windows 7's ascension to the Windows throne in October 2011. According to data StatCounter published over the weekend, Windows 7 accounted for 51% of all personal computer operating systems in August, nearly double XP's 29%.

Even though Windows 8 was released to some users in mid-August and Microsoft has offered a free 90-day trial to the upgrade, its share of all Windows moved only slightly last month, from 0.2% to 0.3%. The latter number represents 30 out of 10,000 PCs, or 3 out of 1,000.

OS X Mountain Lion's share of all Macs jumped to 20%, while Lion plunged to 34% in August. (Data: Net Applications.)

Microsoft won't officially launch Windows 8 -- and computer makers won't start selling Windows 8 PCs and tablets -- until Oct. 26.

Net Applications measures operating system usage by tracking unique visitors to approximately 40,000 Web sites.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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