It would be nice if we could expect people to always do the right thing (even in the face of confusion over what that is due to poor UX), but ultimately you hav

It would be nice if we could expect people to always do the right thing (even in the face of confusion over what that is due to poor UX), but ultimately you have to deal with the reality that humans are flawed and the more humans you add to the mix, the more likely one bad (or confused) apple will do the wrong thing. If your platform exists to form links between these (billions of) people, you have to either be extremely proactive with user's privacy (which Facebook clearly isn't), or you have to educate them to accept that anything they post should be treated as if it were public to the world, because basically it is (via 6 degrees of separation combined with Facebook sharing rules).