Everybody know Facebook username system. In short username is the personal URL which Facebook gives for direct access to Facebook Fan pages and profile.
First FB was given it any how without any thinking but due to increasing FB demand and its importance FB has stop allowing creating personal URL or username(see the below example).
Eg: In the past if people have a page containing cookies shop. they would create a page username as www.facebook.com/cookies
Facebook does not allow any username to change once it is decided similarly(However Now it allow stating you can change your username only once). Even if the shop runs or not the page with that username is permanent and no one else can change it hence it has increase the procedure's.