I speak negatively about Facebook and privacy so much, I am happy to have a chance to say something positive.
Recently, in France, a rumor started that Facebook is sharing private messages on public walls. I am not sure what caused the rumor to start, but I can say, that this did not happen.
My favorite debunking site, Snopes, researched the issue at length. What has happened is that people forget about old wall posts, see them all of a sudden because of the new Timeline, and freak out. Wall posts used to look a bit different back when Facebook first started, and people would create posts back and forth to have conversations. Given how Facebook works now, this looks like a series of private messages. But no, it is just how people used Facebook back then.
Anything that can be “liked” is a post. Private messages cannot be liked. So if you think a private message has been shared, check and see if you can like it. If you can, you posted it as a regular status update, and just forgot about it.
It is easy to forget how much we share on Facebook sometimes. It never hurts to go back to the earlier days and see if you are happy with what you posted back then. It can be time consuming, but it might be worth it.