I don't have a facebook account but idk about this. Will users get another username for their email address or use their vanity usernames? The under 18 crowd wi

I don't have a facebook account but idk about this. Will users get another username for their email address or use their vanity usernames? The under 18 crowd will use it because like another poster brought up, my brother also doesn't have a email address. He tried using mine.