Facebook notified users on Sep. 28, 2018 that it discovered a security issue affecting almost 50 million accounts. A vulnerability in the “View As” feature, a feature allowing Facebook users to see how their profile looks to someone else, allowed cyber thieves to steal access tokens.
The affected users have been logged out of their account by Facebook, forcing users to generate new access token upon subsequent login. Users who use Facebook to login to other accounts and services may also be impacted. Facebook has temporarily disabled the “View As” function, while it is completing a thorough security investigation.
Read more about it here.