Facebook uses 64-bit integers (bigint) for their user ids. So you use Bigint UNSIGNED in MySQL.
“As a reminder, in the very near future, we will be rolling out 64 bit user IDs. We encourage you to test your applications by going to www.facebook.com/r.php?force_64bit and create test accounts with 64 bit UIDs.”
Edit: Facebook usernames is not the same thing as the user id. The username is of course varchar but will not be returned as the id.