You're assuming that you are the one doing the posting. If my parents and my sister are on Facebook (they are), I'm in their address book and if that got upload

You're assuming that you are the one doing the posting. If my parents and my sister are on Facebook (they are), I'm in their address book and if that got uploaded to Facebook (quite likely), I'm there now despite the fact I don't have an account. Plus, you can be tagged in photos as well. Plus, they have their cookies which allow tracking of you across large parts of the web. Note that in all of this, I'm not using Facebook at all and not even visiting it. But, I'm in their system with data about me that I didn't put there.