I think the market has changed the way it tracks statistics for older phones.
I see a similar drop by your February 14th. If I break it down to an Android version, I see that versions 2.3.3 and later remain unchanged, while older versions like 2.1 show a sudden drop. I also see a clear drop for older phones like the Motorola Droid, while newer phones like the Galaxy S2 don't fall. 
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