I was pretty surprised how many sites use Quantcast... I don't personally use it (I don't see the upside really). But overall, 8% of all sites use it. Which is nothing compared to the 67% that use Google Analytics, but it still puts it in 2nd place as far as analytics usage.
I use quantcast, and in fact, just got an email from them today saying that they are now using direct first-party cookies and third-party cookies, instead of just third-party cookies. They've changed the historical data to match the change in tracking methodology as well. Might be worth checking out, they say most statistics should go up!