https://www.pcgamer.com/50-million-facebook-accounts-affected-by-security-vulnerability/

Facebook has issued a statement warning its users of a "security issue" that affects nearly 50 million users accounts. An investigation is underway, but Facebook that "it's clear that attackers exploited a vulnerability in Facebook's code." 

That vulnerability arose from the "View As" feature that enables users to see what their profiles look like to other people. Exploiting a change made in July 2017 to the video uploading system, hackers could take control of Facebook access tokens—"the equivalent of digital keys that keep people logged in to Facebook so they don’t need to re-enter their password every time they use the app"—and use them to take over connected accounts.