An interesting point is made here: http://www.shinyshiny.tv/2012/09/facebook-private-message-le... "... many users who are still adamant the messages they saw were private and claim that Facebook is just trying to cover up its mistake by calling them all crazed liars."
I guess I am in that group, I strongly believe some of the messages I saw were originally private. Unfortunately, there is no easy way to prove this.