Every time I visit a page with a little FB "Like" button on it, Facebook knows (via a cookie) what page I've visited. They track people even if they don't have accounts. It's not so easy to opt out of that. (I installed ShareMeNot for Firefox but it's not obvious that you can do that.)
Edit your hosts file.
That only works until FB (or Google or whoever) starts using IP addresses in their CDN URLs, or Google hard-codes DNS values for Google analytics servers into Chrome, or whatever. ShareMeNot prevents the browser from even trying to fetch the content.
Well if "Google hard-codes DNS values for Google analytics servers into Chrome", your precious ShareMeNot is as useless as my suggestion.