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+
Even if you have explicitly signed-out of Facebook, the presence of the like button on a site allows Facebook to track your movements on that site. Their like button javascript reports back a unique login cookie that is not erased when you sign off of Facebook. Pressing the like button is a small part of its purpose. Its purpose is to track you around the web as long as you have ever logged into Facebook before. This was all covered here on HN months ago, when I first learned of it.