The real comparison is whether ads placed on social sites do worse than Google's display (AdSense) networks.
If they do (and it sounds like they do), it's likely because the majority of social sites don't show up in search engine result pages (SERPs). My understanding is that most/many of the clicks on AdSense come from people clicking search results, and then seeing an ad relevant to their on one of the pages they click through.
Display will never equal search; intent isn't there. The only real money to be made in display is a) via SERPs and b) branding/influence ads, whose effectiveness is not easy to measure, since it isn't about clickthroughs.
And the social sites will need to move either to brand ads or to truly social ad concepts (ala facebook fan pages) that become part of the app’s experience and don’t draw users offsite.