There are apparently a lot of user accounts being created on Stack Exchange sites who are named
facebook- followed by their numeric Facebook user ID.
There are over a thousand such users on Stack Overflow: https://stackoverflow.com/users?tab=Reputation&filter=all&search=facebook-
I'm guessing this is a bug in the feature to log in with Facebook? Presumably it should be using their real name, or a generated default name. Naming people after a website is a bit mad.