A software bug meant millions of Facebook users may have unknowingly posted private information to the public, the company has warned.
The glitch set a user’s post to be shared to "everyone", even if a user had previously chosen a more restricted option, such as “friends of friends”.
“We’d like to apologise for this mistake,” said Erin Egan, Facebook’s head of privacy.
Users who may have been affected will be notified on the site’s newsfeed.
"We recently found a bug that automatically suggested posting publicly when some people were creating their Facebook posts,” Ms Egan said.
"We have fixed this issue and starting today we are letting everyone affected know and asking them to review any posts they made during that time.
"To be clear, this bug did not impact anything people had posted before - and they could still choose their audience just as they always have. We’d like to apologise for this mistake."
The glitch was active between 18 and 22 May, a spokeswoman added, but it took the site until 27 May to switch posts back to private - or whatever the user had typically used before the bug became apparent.
Mark Zuckerberg is Facebook's chief executive