Comscore Media Metrics reports came out earlier.
http://www.comscore.com/content/down...rch%202012.pdf
Table 4 is always helpful.
It lists the top ad networks for the US (the ones with ** are ad networks)

Top 50 Ad Focus Ranking
Google Ad Network led the March Ad Focus ranking with a reach of 91.7 percent of Americans online, followed by AOL Advertising (83.1 percent), Google (81.7 percent), Yahoo! Network Plus (81.4 percent) and AT&T AdWorks (81.1 percent).
Now here is the bit to consider
With facebook only having a 70% reach of US online population, its on position 16
And with all the account trouble lately -> go and diversify!
Good stuff