You can create a custom Facebook address for your personal account or Business page such as for my personal account I can direct people to www.facebook.com/kierandmoore and for our Business page www.facebook.com/imokilly.
This make it easy to tell people about your page or profile when in conversation or just sending a link. To be eligible for a username you must have a least 25 friends or 25 fans for a business page. This is to prevent people from creating accounts just to block a username been taken. Username must be a least 5 characters long.

Creating a username for your personal account or business page:

Go to www.facebook.com/username. Here you will be asked to verify your account if you have not already done so.

As you can see I have already selected a username for my personal account. To do this yourself it will give you some suggestions or you can enter your own. It will then check the availabilty of your username. If it is free it will ask you to confirm your choice and warn you that once a username has been chosen it cannot be changed. Check you name for spelling and be 100% sure this is the username you want. This is very important as there is no way to change this later.

The second box will be displayed if you are an administer of any Facebook Business Pages. If you administer more than one they will display in a drop down list. Select the page you would like to add a username for and enter the name you wish to choose. Facebook will then check the availabilty of that username.

If the username is available you will be shown this screen. Double check your selected username for spelling and click confirm once you are happy with it.This is very important as there is no way to change this later.