Pew reports 12% penetration of ereaders in the USA. Survey-based, but believable. Of course, Amazon has never reported Kindle sales (and nor has Barnes & Noble), so this is the best we can do. 

The interesting thing is that ebook penetration in the USA is also around 15% (up from 10% at the end of last year). Yet, typically 10% or so of the population buys 25% of the value of book sales. So if it was simply that the biggest book buyers were buying ereaders and converting their purchasing 100% to ebooks, you’d expect ebook sales to be more like 20-30% of the US market. That isn’t the case. 

Of course it is possible that this discrepancy can be explained purely by a mix of lagging use and unreliable statistics. But more likely, it implies that the sales of ereaders are to a mixed group of readers, or that people who buy ereaders continue to buy print books, or both.