It is still unclear how numerous 14 million profiles that were affected may have posted content privately without realizing they were sharing publicly. People could have changed the individual audience setting on posts, but would have had to notice the setting was different from what they'd chosen.
Facebook says it didn't make the posts private until May 27, so it's possible that mere acquaintances could have seen sensitive info during that nine-day span.
"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", Erin Egan, Facebook's chief privacy officer, said in a statement. "We'd like to apologize for this mistake".
When posting on Facebook, users have the ability to specify who can see their posts.
After the bug was detected, Facebook engineers spent 5 days resetting posts created by affected users to private, even if that was the original intent. The affected users will then be shown which posts were marked public during the glitch.
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Facebook posts typically default to the last "audience" a post was shared with, such as family members, friends, or friends except their boss.
Affected Facebook users received a notification on the app or website starting Thursday. During that time, the privacy settings on some users' posts were changed to public without any warning. That way, users can reset a post that was inadvertently set to public back to being shared just with friends if they would like.
The news follows a recent furor over Facebook's sharing of user data with device makers, including China's Huawei.
Facebook said it estimates 14 million people did so - and so has started notifying users.
A spokesperson told the BBC this method of communication might become more frequent as the network works on ways to be more transparent with users.