The latest numbers from comScore Networks continue to illustrate the challenge facing Microsoft in the mobile market. On a three-month rolling average, the company’s share of U.S. smartphone subscribers fell to 5.8 percent as of May, down from 7.7 percent in February, according to a new report from the research firm earlier today. See this chart below.

But it’s the long view where the trend really becomes clear. Digging into the archives, comScore’s numbers from a year ago show Microsoft’s current 5.8 percent comparing to 13.2 percent as of May 2010.  At the time, that was just ahead of Google — whose Android operating system has since soared to the No. 1 position with more than 38 percent of the market. Here’s that chart.

Going back just a little further, Microsoft stood at nearly 20 percent of the market less than two years ago, in October 2009. That was No. 3 — behind RIM’s BlackBerry in the No. 1 spot (41.3 percent at the time) and Apple’s iPhone (24.8 percent). Here’s that chart.

The entire market has changed dramatically since then, as shown in the first chart at the top, with RIM slipping to the No. 3 position in the latest comScore report, at 24.7 percent as of May 2011, behind Apple in the No. 2 spot with 26.6 percent.

Microsoft’s next update to Windows Phone, dubbed Mango, has been getting positive reviews. The company at this point is banking on the possibility of reversing its fortunes over time — hoping that the market remains just as dynamic, but in its favor instead.

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