The question of how many Windows Phone 7 handsets are out there still remains unanswered, but what we do know at the very least is that the number of active Windows Phone 7 users of the Official Windows Phone 7 Facebook app has crossed the 500,000 mark.
While the number is still far off from the 75 million on iOS or 36 million on Android, it is not far off from Facebook for webOS, which only has 672,000 users, and in fact dropping rapidly.
Compared to the other major mobile operating systems and the number of users of their official Facebook apps, it is clear that the Windows Phone 7 installed base is still pretty small, less than 1.5% of the size of Android and Blackberry and 0.7% of the iOS devices.
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On the other hand over the last 30 days Windows Phone has been growing faster in relative terms than Android, and certainly a lot faster than the iPhone and Android. And of course we expect a major boost in absolute numbers when Nokia introduces devices in later 2011 and early 2012.