Demographic info may not be the only thing they know about. Given a lot of sites embed some sort of Facebook widget on their pages, they have a tracking mechanism.

If I had not explicitly signed-out of Facebook before visiting all these other sites, Facebook knows about it. And maybe also the browsing habits of people in my network. That gives a really deep profile about me - a lot more than just my demographics.

Recently there was an excellent article on ReadWriteWeb, about why Facebook terrifies Google: http://www.readwriteweb.com/archives/why_facebook_terrifies_...

Check out screen shots of their respective ad platforms in that article: the level of targeting that is possible with Facebook (which is only going to become deeper, given all the additional details they are gathering about me) is simply not possible with Google. This is one major reason why Google is pushing hard with Google+