In the past, you were required to get 25 likes for your Facebook page in order to get a custom URL. That’s no longer the case since September 2011. There are no requirements other than simply creating a page.
Just go here to get your custom URL. http://www.facebook.com/username/
Why do this?
Simply because shorter is better for URLs
A URL (Universal Resource Locator) is set of characters (all that the http://, www, .com stuff) that points to a “resource” on the Web. That resource can be a file, image or other.
When speaking about your website address it we often use “link”, “website”, “address”, and “domain name” interchangeably to refer to the same thing – a URL.
A simple, branded, clean looking and short Facebook URL makes things look clean and professional.
A shorter URL is always better for use on the Web because it’s less likely to get cut off in email messages, text messages, forms or other uses where spacing is limited.
Default Facebook page URL (BEFORE)
Custom Facebook page URL (AFTER)
Don’t worry, you’re old Facebook URL will still continue to work.
Once you set up your custom Facebook URL, your old longer, uglier one will still continue to work. Thus, you don’t need to go back and edit any places you have used your old link.
However, if your old longer link is used in emails messages (e.g. a newsletter), then you should go update them because email messages often get truncated which risks breaking up your long URLs into two links and messing them up.