If Microsoft (MSFT) was hoping that the launch of Windows 7 would halt the erosion of its operating system market share — and curb further inroads by Apple (AAPL)  — there is no evidence that it’s working yet.

In fact, preliminary data released overnight Sunday by Net Applications show Mac OS X’s Internet share growing by 2.73% in October, from 5.12% to 5.26%.

Windows’ Internet presence, meanwhile, fell from 92.77% to 92.54% — its ninth loss in 12 months. Windows 7’s share, however, was more than 2% even before its Oct. 22 general release, thanks to widespread use of early release versions. By Oct. 30 the Windows 7 portion was 2.85%, largely at the expense of Windows XP, according to a separate Net Applications report.

Net Applications, it must be noted, is not measuring share of market in the sense of sales revenue or unit sales. Rather it tracks the presence of various operating systems on the Internet by sampling browser data from visits to its clients websites — some 160 million hits per month. It’s a methodology that tends to favor devices that make it easy to navigate the Web, which explains the relatively high “market share” of the iPhone in the firm’s monthly surveys.

You can see Net Applications’ October report here. The results are summarized in the chart below.

[UPDATE: Net Applications issued an update Monday entitled “Windows 7 breaks 3% in daily tracking.” Meanwhile, their quality control department has reviewed the OS market share trends shown below. In the revised table Windows’ market share dropped a bit (to 92.54%) and Mac OS gained (to 5.27%). See here.]

See also:

• Mac’s Internet share grew 5% in Sept.
• Net Applications: Apple just lost half its ‘market share’

[Follow Philip Elmer-DeWitt on Twitter @philiped]