Facebook has stated this clearly in its blog:

Starting at 12:01 a.m. EDT on Saturday, June 13, you’ll be able to choose a username on a first-come, first-serve basis for your profile and the Facebook Pages that you administer by visiting www.facebook.com/username/. You’ll also see a notice on your home page with instructions for obtaining your username at that time.

According to your user name, the choices will appear in the vanity URL setting page. You will be allowed to choose only one from the list. Apart from this, it also offers an option to enter a name of your choice but it must be at least five characters and only include alphanumeric characters.

Organizations, celebrities and politicians are advised to claim their URL early to avoid cyber-squatting.

Susan Jacobson, of Temple University’s journalism department, says “Waiting for a service to catch on or to become newsworthy is waiting too long. Cyber-squatters will start registering names as soon as they become aware of a new service. While it is possible to get your name back, it will take some time and could be costly.”

Getting a vanity URL will help you get huge traffic via natural search. Don’t forget to grab yours now.