What is a Facebook username (or "vanity URL)?
Facebook business or fan pages can be customized to have a unique URL. This is sometimes called a "vanity URL", but Facebook now typically calls it a "username". This way, rather than a string of letters/numbers such as facebook.com/page/mycompany/12548785, it can be something simpler, such as facebook.com/mycompany. You must be an admin of the Facebook page to make this type of change.
Why do you need it?
Setting up a username is optional, but it gives you a shorter, more memorable web address for your business page. It also makes it easier to add your page into Feeds (though you can still do it using the string of letters/numbers, if necessary).
How do you claim it?
1) Go to www.facebook.com, click the drop-down menu in the upper-right corner, and make sure you are switched over to your business/fan page.
2) Go to your business's current Facebook page, and click the "About" button in the left-side menu.
3) Click "Edit" next to "Username", which is listed under the "General" section.
4) Enter your desired username (which will become your URL) in the username field. If the username is not taken by someone else, a green check mark will appear. You may need to adjust your username slightly if it is already taken.
5) Click "Create Username", and you are finished.
Now that you've claimed your vanity URL, you are ready to add your Facebook account as a source in Feeds.
Vanity names can only be changed once, so think carefully about your choice.
You can’t transfer the ownership of a username to another party.
Usernames can only include alphanumeric characters (A-Z, 0-9) or a period (.).
You can only have one username per page.
You can’t violate anyone trademark rights.
Don't bother acquiring a username to sell it in the future.
Some generic words are not available.
Notify Facebook if your trademarked name has already been taken.