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. You’ll also see a notice on your home page with instructions for obtaining your username at that time.
Custom URL for Facebook
Think carefully about the username you choose. Once it’s been selected, you won’t be able to change or transfer it. If you signed up for a Facebook Page after May 31 or a user profile after today at 3 p.m. EDT, you may not be able to sign up for a username immediately because of steps we’ve taken to prevent abuse or “squatting” on names.
Visit : www.facebook.com/username
Source : Facebook Blog.