Facebook un-blocked users that had been blocked by hundreds of thousands of people.
The vast hack could have exposed Facebook users to danger from people they thought had been banned from seeing their posts.
Facebook's blocking tools allow people to stop a user from seeing things on your profile, messaging you on Messenger or adding you as a friend. But a bug meant that stopped working, allowing blocked users to see people's posts and get in touch with them on Messenger.
Blocking can be an important way of ignoring and hiding from everything from annoying users to abusive ones. Facebook noted that the feature is often used for reasons that are "more serious like harassment or bullying".
Facebook said that the problem had affected 800,000 of its users. But some of those people had more than one person unblocked, meaning that problem could have affected more problem users than that.
Anyone affected by the problem will receive a message that informs them of the bug and encourages them to check their block list. It also sends people to Facebook's safety hub, which offers tips on how to deal with harassment or abusive users.
The problem lasted between 29 May and 5 June, Facebook wrote in a blog post. It has now been fixed and everyone has been re-blocked, it said.
Of the 800,000 people affected by the issue, 83 per cent had only one blocked person un-blocked. Facebook also said that the bug didn't re-instate any friendships that had been broken during a block, and that the problem didn't allow blocked people to see posts that had only been shared with friends.
