Microsoft's share of the smartphone market is plummeting at an alarming rate--so much so that the company's last ditch effort to make an impact in mobility, Windows Phone 7, may be irrelevant by the time it manages to ship the much-anticipated Mango update and realize its partnership with Nokia later this year.

Data released Thursday by comScore shows that Microsoft's average share of the U.S. smartphone OS market over the three months ended in June came in at just 5.8%, down from 7.5% from the three months ended in March, and down from 8% for the three months ended in January.

The last number represents the first, full three-month period, as measured by Comscore, in which Windows Phone 7 devices were available--meaning Microsoft's share of the smartphone market has fallen 38% (from 8% to 5.8%) since the Windows Phone 7 launch. That's doubly troubling for Redmond, because the numbers also include Windows Mobile devices that are still in use. Actual sales are difficult to measure as Microsoft does not release such data.

Windows Phone 7 devices first became available from a range of manufacturers, including Samsung, HTC, LG, and Dell on Nov. 8, 2010, on the AT&T and T-Mobile networks. Sprint and Verizon subsequently picked up the platform.

The question is for how much longer handset makers and carriers will consider it worth supporting Windows Phone 7. Microsoft's mobile market share has been declining at a compound rate of about 5% per month for the past six months. At that pace, its overall share may be be hovering around just 4% by the end of the year.

Meanwhile, rival Google is on track to dominate smartphones. Android devices held 40% of the market as of the end of June, according to Comscore. Apple's share came in at 26.6%, while RIM's share, also in decline, fell to 23.4%.

Microsoft is hoping to gain some ground when it introduces an updated version of Windows Phone 7, dubbed Mango, later this year. Mango adds 500 new features to the platform, including multitasking and hardware-accelerated Web browsing through Internet Explorer 9, according to Microsoft.

Mango's debut should also coincide with the arrival of the first Nokia phones running Windows Phone 7, though Microsoft has yet to provide precise arrival dates for Mango or Nokia phones. Under a partnership announced last year, the Finnish phone maker is transitioning its entire smartphone line to Microsoft's mobile OS.

Whether Windows Phone 7 is a legitimate player in the market by the time that happens remains to be seen. The current numbers suggest otherwise.

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