Of any single device? I'm not sure, all combined maybe. The B&N nook alone is probably the most common one but probably only a half million units sold. So even all combined probably not.
The nook has such little power that to call it "Android Powered" is a gross misuse of the term. Yes, you can downgrade the firmware, root the device, and upgrade it, but the web browser is still god-awful and I didn't see any good third party apps.
For an e-book reader, with the option of playing Chess and MP3s, its top-notch. But right now, the ability to run apps on it is more theoretical than practical.
