Facebook Username is customized URL of Facebook Profile. After registering an account, each and every user gets a unique ID (http://www.facebook.com/profile.php?id=100003663899133). Here “id=100003663899133″ is unique id, which you can customize with unique name and extension.
To get Username, you need to provide your mobile number to verify your account. Facebook will send you SMS which you need to verify. If you don’t have mobile number then it will take longer time to get username.
To get Facebook username, go to Account Setting, on the right hand side Click on Edit button to customize the username. Alternatively, you can visit http://facebook.com/username too.
Remember you can only change your Username once. After having your username your profile URL will be structured as (https://www.facebook.com/yourusername) . You username will be set as your Facebook email (eg: firstname.lastname@example.org). Now, onward you can login Facebook account with username.