Facebook said to The Verge:
"A small number of users raised concerns after what they mistakenly believed to be private messages appeared on their Timeline. Our engineers investigated these reports and found that the messages were older wall posts that had always been visible on the users' profile pages. Facebook is satisfied that there has been no breach of user privacy."
Some time after, a company spokesperson said to TechCrunch, and later confirmed to The Verge:
"Every report we've seen, we've gone back and checked. We haven't seen one report that's been confirmed [of a private message being exposed]. A lot of the confusion is because before 2008 there were no likes and no comments on wall posts. People went back and forth with wall posts instead of having a conversation [in the comments of single wall post.]"
At this point, it seems far more likely that users are simply reading the news stories about possible privacy issues, digging back through the social network, and finding information that "seems" like it would've been private. Plenty of these users are posting images of the summary boxes that appear when you look back inside Facebook's Timeline profile pages (pictured above), but no example we've seen is at all convincing enough to corroborate claims of a privacy breach. It's worth noting that France, where the story originated, is a country where Facebook's Timeline profile pages (which afford a larger degree of freedom looking back to previous posts) were recently forced upon users who hadn't yet switched.
After all, a lot has changed since 2008 — for one, people mostly post links to each others' walls. Back then, people spent a few minutes composing wall posts to each other, and thus appear a lot more "private." Also, once wall posts were published, nobody had the ability to like or comment on them, so they inevitably look like messages when looking back through Timeline. Additionally, Facebook's private messaging product was in its early stages, and didn't exist on mobile.
When Facebook launched Timeline at the end of 2011, a similar outcry erupted from users angry that their old posts, while always public to friends, were now a bit more public with the help of Timeline's easily navigable new profile. Before Timeline, it was very difficult to dig back into a friend's wall and see what others had posted weeks or months before. So, a "feature" became the cause of an alleged privacy breach. Until we see evidence — a side-by-side screenshots of a private message on a Timeline and in a user's Messages folder — this seems like another episode of users upset by Facebook unearthing more of their own very public past.
Perhaps Facebook needs to do a better job making clear exactly what posts from your past are now more visible, but in this case, it seems that the culprit for user confusion is the quickly evolving way we talk to each other online.