https://www.cnet.com/news/facebook-says-millions-of-instagram-passwords-were-exposed-internally/

Facebook typically hashes and encrypts passwords so that even its own employees can't see them. That helps ensure that user passwords are protected and doesn't land in the wrong hands. The company discovered that hundreds of millions of passwords were stored in plain text after a routine security review in January. 

Facebook, which has more than 2 billion users worldwide, is also notifying hundreds of millions of Facebook Lite users and tens of millions of other Facebook users that had their passwords exposed internally.