When visiting Facebook, they set their cookies....if you don't clear your cookies after leaving the Facebook site (even if you log out) their cookie continues to track your every move....then they serve up ads to you.

Almost everyday I leave the cookies on place for a few minutes and surf to ludicrous sites that I have absolutely no interest in...then I delete the cookies...and let them collect bad data, sell it, and then serve it up on the pages....probably not a nice thing to do as far as the advertisers buying slots go, but it does give me some satisfaction.