ComScore just came out with a press release that says 20% of all online display ad impressions are happening on social networks. And 80% of that is coming on MySpace and Facebook:
There’s very mixed evidence regarding the efficacy of advertising on social networks. And among the social networks there’s widely varying inventory/impression quality. Indeed, social networks have contributed to the deflation around CPM rates because their massive page views have flooded the market with impressions.
For its part Facebook is experimenting with lots of different types of ad targeting: demographic, marital status, location, even birthday.
Related: Summarized by eMarketer here are some of the barriers to marketing on social networks (Equation Research):