Facebook admits more than 800,000 users may have been impacted by a bug that unblocked people they had previously blocked.
The company says the bug was active between May 29 and June 5, 2018.
Facebook says the problem has been fixed.
This is the second software bug in less than a month the company has notified users of.
Facebook said in June that a software bug led some users to post publicly by default, regardless of their previous privacy settings.
That bug impacted as many as 14,000,000 users over several days in May.