I mentioned this in another comment too: Nobody seems to reads the actual posts, just the headlines. They were accidentally stored in logs:

As part of a security review in 2019, we found that a subset of FB users' passwords were temporarily logged in a readable format within our internal data systems,

which is something I've seen at other companies too. For example, if you have error logging that logs the entire HTTP request when an error happens, but forget to filter out sensitive fields.