Some user passwords were being stored in a readable format within our internal data storage systems. This caught our attention because our login systems are designed to mask passwords using techniques that make them unreadable. We have fixed these issues and as a precaution we will be notifying everyone whose passwords we have found were stored in this way. We estimate that we will notify hundreds of millions of Facebook Lite users, tens of millions other Facebook users, and tens of thousands of Instagram users. No evidence to date that anyone internally abused or improperly accessed them.”]
Source: https://about.fb.com/news/2019/03/keeping-passwords-secure/