A technical bug accidentally turned off blocking controls for 800,000 users as shared below between May 29 and June 5, 2018 as shared below

Letting People Know About a Blocking Bug

Starting today we are notifying over 800,000 users about a bug in Facebook and Messenger that unblocked some people they had blocked. The bug was active between May 29 and June 5 — and while someone who was unblocked could not see content shared with friends, they could have seen things posted to a wider audience. For example pictures shared with friends of friends. We know that the ability to block someone is important — and we’d like to apologize and explain what happened.

When you block someone on Facebook they cannot see things you post on your profile, start conversations with you on Messenger or add you as a friend. Blocking also automatically unfriends them if you were previously friends. In the case of this bug:

* It did not reinstate any friend connections that had been severed

* 83% of people affected by the bug had only one person they had blocked temporarily unblocked

* Someone who was unblocked might have been able to contact people on Messenger who had blocked them

This issue has now been fixed and everyone has been blocked again. People who were affected will get a notification on Facebook encouraging them to check their blocked list.