https://motherboard.vice.com/en_us/article/bja7qq/how-50-million-facebook-users-were-hacked

On Friday, Facebook revealed that hackers broke into the company’s servers and potentially stole the data of up to 50 million people.

The social network forced 90 million people—around 50 million victims plus an additional 40 million that may have been affected, according to the company—to log out and log back in again. That’s because the hackers stole their “access tokens,” a sort of digital key that Facebook creates when you log in and allows you to stay logged in when the Facebook mobile app wants to open another part of Facebook inside a browser, for example (this might occur when you click a link.)