Google could still put ads in front of more people than Facebook, but Facebook knows so much more about those people. Advertisers and publishers cherish this kind of personal information, so much so that they are willing to put the Facebook brand before their own. Exhibit A: www.facebook.com/nike, a company with the power and clout of Nike putting their own brand after Facebook’s? No company has ever done that for Google and Google took it personally.
The rest of the article is worth a read as well. It’s a former Google engineer explaining why he left and what’s changed inside the organization in the last two years with the transition to social.