The latest numbers from Nielsen paint a gloomy picture for Microsoft and its Windows Phone/Windows Mobile platforms.

In the third quarter of the year, both Windows Mobile and Windows Phone combined held 6.1% of the U.S. smartphone market. That's down 33% from the 9% of the U.S. market Windows Mobile/Windows Phone held in the previous quarter.

Breaking down who owns what of the (sliver-esque) Windows Phone 7 market, HTC leads with Samsung, "other", and Motorola trailing.

HTC's WP7/Windows Mobile devices account for 77%--or the vast lion's share--of all U.S. Windows Mobile/Windows Phone devices. Samsung has the next largest share with 9.8% of the Windows Phone devices in the U.S. The "other" category holds 6.6%, and Motorola has 3.3%.

Keep in mind, these figures include Windows Phone 7 plus the legacy Windows Mobile 5.x/6.x devices that are still out there (probably in the form of enterprise handhelds).

You can probably guess what the rest of the U.S. smartphone market looks like.

Android has the largest percentage of U.S. smartphones, growing from 39% to 42.8% from the second quarter to the third quarter. HTC's Android phones alone own 15% of the U.S. smartphone market, Samsung has 10.7%, Motorola owns 10.4%, and "other" (probably including LG, Dell, Huawei, Pantech, Casio, and others) rounds out the Android presence with 7.2% of the U.S. smartphone market.

Apple's share grew a fraction from 28% to 28.3% quarter-over-quarter.

Research In Motion, meanwhile dropped from 20% to 17.8% quarter-over-quarter. Palm and HP's webOS holds a pitiful 2.2% of the U.S. smartphone market, while Nokia's Symbian brings up the rear with 1.7%.

"In the U.S., 71% of those with smartphones own either an Android device or an iPhone," explained Nielsen. "But when it comes to smartphone apps, iPhones and Android smartphones are even more dominant: 83% of app downloaders, that is, those who downloaded an app in the past 30 days, use iPhone or Android smartphones."

The good news for Microsoft and its Windows Phone platform is that the Mango update just began rolling out in late September and early October. In fact, Mango-based phones didn't really hit their stride until mid-October. With HTC's Titan and Nokia's Lumia 800 on the road to being hits, it is possible Microsoft's presence in the U.S. market can reverse itself.

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