Facebook rolled out a new feature few weeks back which allows you to choose your own custom username. Most of the sites locks up your username once you choose one.. you are stuck with it for the life of your stay on that site. But now with facebook, if you are not happy with your username that you selected, you can change it as per your choice..
Users are now able to change their usernames by going to “Account Settings” and looking under “Username.” Facebook is still imposing some limitations, alerting you that “You can only change your username once” when you click the “check availability” link on the new setting.