According to this, "Apple said it had sold 25 million iPads to date."
The Pew questions asked about tablets in the United States, which included more than just iPads (altho iPads surely dominate, probably by 80+%) -- but iPads also sell outside the US. Assuming that the population in the US is on the order of 300 million, 8% would imply about 24 million. If we assume that iPads sold outside the US are about the same in volume as competitor tablets in the US (if you think this is unreasonable, I'd be interested in your reasoning and/or evidence, if any. If this is just duelling assertions, I'll stick with my own.), this suggests that Pew's assessment of tablet penetration is Correct.
If Pew got tablets right, do we think they also got ereaders wrong? If they got ereaders right, 12% implies that there are 36 million ereaders out there, give or take. Given that Digitimes has reported over 12 million shipped in 2010 and over 25 million expected to ship in 2011, 36 doesn't sound entirely wrong.
I'm a little puzzled, because there have also been reports that more media tablets shipped in 2010 than e-book readers. I'm not sure exactly to make of all this, other than to note that just because something ships does not mean it sells. My erratic skimming of The Digital Reader suggests that there's a fair amount of rebadging going on with ereaders, and there are a _lot_ of crappy, cheap tablets that aren't worth buying even at a very low price point.