Apple's Mac OS X's market share continued to grow at Windows' expense. For the second month in a row, Mac OS X posted a record increase, growing by 0.76 of a percentage point to end the month at 9.6%. December was the first time that Net Applications had pegged Apple's operating system above the 9% mark.

And just as Windows set a record for a two-month drop, Mac OS X set a record for a two-month increase during November and December. Those months' combined gain of 1.4 percentage points was substantially larger than the earlier record, a 0.9 percentage point boost the operating system received in September-October 2006, and almost double the 0.73 percentage point increase of November-December 2007.

Mac OS X's market share was up 2.3 percentage points during 2008, an annual growth rate of 31.7%.

Net Applications again attributed some of the decline of Windows and the corresponding growth of Mac OS X to special circumstances. "The December holiday season strongly favored residential over business usage," the company said on its Web site. "This in turn increases the relative usage share of Mac...and other products that have relatively high residential usage. All December usage statistics should be read in that context."

Last month, Net Applications' executive vice president of marketing, Vince Vizzaccarro, said November's drop in Windows share was due to the higher-than-average number of weekend days and the Thanksgiving holiday in the U.S. Net Applications has consistently reported that Windows' share slips during times when browsing is conducted from non-work computers, where Microsoft's operating system dominates even more than in homes.