A rumour that Facebook users' private messages were appearing on public timelines is false, the firm has said.
Some users, mostly in France, reported that "private messages" sent between 2007-2009 were suddenly public.
However, Facebook told the BBC: "[The] messages were older wall posts that had always been visible on the users' profile pages.
Separately, Facebook shares fell up to 11% after an influential journal said the stock is worth just $15.
On the messaging rumour, the social network said: "Facebook is satisfied that there has been no breach of user privacy."
Another source at the company told the BBC that engineers said there was "no way" the two areas of data could get mixed up.
The source said that "no mechanism" had ever been created that would allow a private message to be published onto a user's wall or timeline.
Similar rumours surfaced and were investigated in 2011, the source said, but after "extensive investigation" they were proven to be untrue.
Despite Facebook's statement, many users reported finding what they believed to be old private material.
"There are messages I've got on my wall that are most definitely private messages! From 2006 all the way up to 2012," wrote Poppy Dinsey, owner of fashion social network What I Wore Today.
"I'm 100% sure by content and from memory. And also some posts on my wall were clearly beginnings of (private) group [messages]."
Meanwhile, Facebook shares were under pressure after the US financial publication Barron's said the stock is worth perhaps only $15 a share, well below the $38-a-share flotation price.
On Wall Street, the shares ended down 9.1% to $20.79, having fallen more than 11% earlier in the day.
Barron's said: "Facebook's 40% plunge from its initial public offering price of $38 in May has millions of investors asking a single question: Is the stock a buy?
"The short answer is 'No.' What are the shares worth? Perhaps only $15," Barron's said.