When facebook started allowing us to choose a user name for the url they stated that you could not change it. Today you can change it, but supposedly only once. Simply go to settings and click on user name an change it. I tried to do it for a couple of new accounts to select a user name url, but it gives me an error message. I heard today that people between 18-24 spend an average of 40% of their time on social sites. That makes my black hat tingle.