Mass logout occurred at approximately 8:30 PM EST on Friday night. Facebook said they are aware of the issue and believe a configuration change caused the mass logout. Facebook is working to get things back to normal as quickly as possible. There is no indication at this time that a security breach occurred. The majority of users logged out were using a mobile device, according to reports on Twitter and DownDetector. Facebook shared the following statement with us Saturday morning: “We investigated the issue and fixed it for everyone earlier today”””
Source: https://www.bleepingcomputer.com/news/technology/facebook-users-were-mass-logged-out-friday-by-configuration-change/