True, but also consider how those two sites evolved. When I post a tweet, I know that it is essentially the same as me taking out an ad in the paper or shouting it at people in the pub. I'm fairly careful with regards to what I post. When I started using Facebook, it was very much about communicating with the group of people you have approved as friends. Over time it has transformed into something much more public, I believe without the bulk of the userbase realising.