Facebook disclosed a security vulnerability that affected 50 million people on the social media network and allowed malicious third parties to potentially access the affected users account. Around 90 million people will now have to log back in to Facebook, or any of their apps that use Facebook Login. Facebook has stated they will provide more information when the investigation has been completed. It is not known how many account, if any, were affected by this vulnerability. The vulnerability stemmed from a change they made in July 2017 to their video uploading feature.
Source: https://www.bleepingcomputer.com/news/security/facebook-vulnerability-affecting-50-million-users-allowed-account-takeover/