All Facebook users receive a unique URL, which can be given to friends or posted on external sites on the Web. Made up of a user name and a user identification number, a user’s profile URL is visible to anyone with profile-viewing capability. You can easily locate a friend’s user ID in a few quick steps.
Log in to your Facebook account.
Click on the name of a friend, which will bring you to that friend's home page. Choose the “Profile” option.
Locate the URL at the top of the profile page. The user ID is the set of digits at the end of the URL following “id=.”