https://finance.yahoo.com/news/facebook-reveals-50-million-accounts-affected-security-breach-165738742.html

“This allowed them to steal Facebook access tokens which they could then use to take over people’s accounts,” Rosen continued. “Access tokens are 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.”

According to Rosen, Facebook has already fixed the flaw and reset the access tokens for the 50 million impacted accounts. The company has also reset the access tokens to an additional 40 million accounts that were the subject of a View As look up as a precautionary measure. In total, 90 million users will have to log back into their Facebook accounts the next time they open the Facebook app or use an app that uses a Facebook login.