Well, if I remember correctly, they track your web navigation using that little like button on every page (even if you don't click it). Then there is the issue that its working on identifying exactly who you live with, where you work, who you socialize with etc. These are things that i can think of right now, and im sure there are a lot more.
Edit: While you where talking mostly about content of fb, i see these issues as being the biggest problem for me personally. I was on the internet in the late 90s and we learned early on never trust anything written on the web, and i apply the same to FB/Twitter/HN/Reddit etc.
