From the beginning, your Facebook Profile URL or Fan Page URL is quite long and so hard to remember. A short url is easier to remember and promote your name/page or application. It also can be used in your marketing communications, company website and business cards.
To do this, simply visit the url: http://www.facebook.com/username/ and then set a username for your profile page, fan pages or applications respectively.
However, there are some important note you should know:
+ Once you have claimed a username by clicking the “Confirm” button, it is not possible to edit it, or to transfer your username to a different account on Facebook.
+ A Page need to have as least 25 fans to be able to claim username.
+ Just administrator of a Facebook Page can choose/ set username for that page.
This link may help you more: https://www.facebook.com/help/?page=900