Google controls the internet, they're the gatekeepers of pretty much all traffic. Their algorithms determine who receives traffic and who doesn't. You can get your business attention by paying Google for the privilege. Facebook has no such ability yet. Search advertising just works much, much better than display ads.

That said, they do have a lot of users and a lot of explicit data about their users, and they could use that to bootstrap FB into something else. But until they figure that out, it's all just potential.