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).
I chose it. Any app that downloads data from the web needs to be careful with age ratings - Stack Exchange has more than programming questions - it has, e.g. video games, with perhaps more age restricted material. I chose 12+ as a compromise between Apple's desire to have age approprate labelling, and restricting likely users.
