Facbook allows you to have “Friendly” URLs that allow for a nicer way for your URLs to be displayed. This also provides an easy way to remember your pages.
For example, you can now view my Facebook page as http://www.facebook.com/davidzielski, instead of something like http://www.facebook.com/profile.php?id=[some huge number].This is a great thing for people who use Facebook and put their Facebook links on their business cards.
Log in to Facebook and you should have a link to choose your username in the upper left-hand corner — or just go to http://www.facebook.com/username/ and attempt to select your username there.
You may feel that Facebook is not that important to your online presence — but if you change your mind in the future, it might be too late to claim the username you want. But remember, once you pick this address, it cannot be changed.