✔ 最佳答案
each account gets only two chances to set their username. once a username is set, that account cant go back to displaying just the profile ID like it used to. After the second time you've set your username, it stays like that permanently. once a username has been taken, it can never be used by anyone else, ever again. this is because facebook doesn't recycle usernames. if you changed your username on your account for the second time, and want the first username back, forget it. it's gone.
if you don't like the username you've chosen, and this is your second username for that account, then the only thing you can do is delete your account. then open another new account. that's what i'd do. but keep in mind, you will never be able to use that username again from the deleted account, so make that decision wisely.
what you read there about facebook taking back a username for any reason. that's only for their own reasons, and whenever they want to do, however they want to do, whatever it is they're doing. not because you went up to them and asked them to. so that has absolutely nothing to do with this issue we're discussing here.