It would be interesting to find out how many users they're getting from their semi recently launched API (http://stackapps.com). I know my iPhone Stack Exchange client http://sixtoeightapp.com has over a thousand downloads, and won't be counted by Quantcast. Whilst it's more of a complementary access point, other apps on Stack Apps might suffice as the sole access point for passive users (i.e. those who just want answers, as the API is read only currently).