By Philip Elmer-DeWitt
October 1, 2009

Apple’s (AAPL) share of global Internet traffic is just a thin green slice in a great pie of Microsoft (MSFT) blue, but that slice grew more than 5% last month as Windows’ share fell — as it has for eight of the last 12 months — according to data released overnight Thursday by Net Applications.

The Mac’s share in September, as measured by the Net metrics firm, was 5.12% — up from 4.87% the month before — while Windows’ was 92.77%, down from 93.06%.

The Mac’s 5.12% seems quite puny when compared with the 9.77% share Net Applications reported for Mac OS X in May. But that was before the company changed the way it weighs its data in order to more accurately reflect worldwide market shares, as opposed to the U.S.-centric numbers it used to report.

The iPhone’s share also grew in September — to 0.35% from 0.33% — a 6.06% increase from a very small base. In May, before the re-weighting of the data, Net Applications reported the iPhone’s share as 0.6%.

Net Applications’ monthly surveys are conducted by sampling browser data from some 160 million visits to websites operated by its clients. The company describes the results as “market shares,” but they do not actually measure share of market in the sense of sales revenue or unit sales. They used to provide a consistent methodology by which to gauge operating system trends. Now, not so much.

You can see Net Application’s September report here. The results are summarized in the chart below.

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