Facebook has a feature that you can assign your account a unique username rather than using a profile id which you can use as your login instead of facebook email. The username could be assigned through the facebook Username page.
Facebook usernames can be used for facebook login, email accounts, and easy profile linking. So change facebook username wisely.
But recently facebook has allowed you to change the username once even if you have it already set. To change facebook username of your account you will have to visit your Account Settings page and you will see a section called username. Click on change username and you will be able to change to a username that is currently not taken by other facebook users.
Then if you used a unused username you will get a confirmation message that will set the username for your account and your facebook email address.
And remember that this is a one time change only, You might not be able to change it later if facebook decides to. So choose a good username that you can live with and set it up.