Facebook reported a data leak today and forced a large number of accounts to log off as a precaution. Source: NY Times and Facebook.
That NYT article says "The company forced more than 90 million users to log out early Friday, a common safety measure taken when accounts have been compromised."
Additional article from The Hacker News - "unknown hacker or a group of hackers exploited a zero-day vulnerability in its social media platform that allowed them to steal secret access tokens for more than 50 million accounts" and "Facebook has already reset access tokens for nearly 50 million affected Facebook accounts and an additional 40 million accounts, as a precaution"