Google last gave data for Android activations on 12 September 2012, when it said that there had been 500m activations and the run-rate was 'over' 1.3m a day. A week earlier Eric Schmidt gave the same 1.3m number and cumulative activations of 480m. Those numbers are not, obviously, consistent (the daily run rate between the two announcements would actually be 2.86m), but then they're round numbers at scheduled events, so one must make allowances.
Since then, though, silence. Google has repeated the same 1.3m number from time to time, but there is no new data. (As an aside, Google's monthly developer stats for Android device screen sizes haven't been updated since the end of September 2012.)
Arithmetically, if one applies the 1.3m run-rate to the 171 days since the last data fix, one would get 222.3m further additions, for a total of 722m (though not all would still be in use).
That means that if one tries to model cumulative Android activations on the basis of Google's statements, fully 31% of the total is an extrapolation from a data point almost six months old. This is... worrying, if you care about any sort of precision. With Samsung refusing to disclose proper unit numbers and the whole long tail of small OEMs also more or less opaque, getting to a good number for the size and direction of the Android market is an increasingly speculative exercise.
Android activations themselves, of course, are a somewhat murky data point. It has never been entirely clear (to me at least) how devices that are activated twice (for the growing second hand market, for example) are treated. On the other hand the great majority of Android devices sold in China, which are probably a third of total Android sales, come with no Google services installed, including no Google Play, and hence are not even included in Google's activation numbers, since signing into Google Play is what counts as 'activation'. Even some Motorola phones are sold in this state. Meanwhile, a phone sold to someone that doesn't even have a data plan will also, obviously, be excluded from activation figures. Such devices are invisible to Google.
In other words, asking Google about Android activations is a little like asking Microsoft about the Windows install base: it has some idea, but not a very precise one. Over time, with the proliferation of Kindle Fires, Android Car DVD players and all manner of other things, it might be a little like asking Linus Torvalds how many Linux devices there are: how should he know?