User IDs supplied to FBML are 64-bit integers created by Facebook to identify each user. You might notice when you visit your Facebook profile that the URL is structured like this: http://www.facebook.com/profile.php?id=683545112.

The number after the id= is your Facebook ID. When planning your app, you may want to store this ID in a database for later reference. In MySQL, I use the BIGINT(20) data type to do this. You will want to find the equivalent for your database environment.

In addition to the ID, whenever there is a uid attribute in FBML, you can use the following identifiers in place of the actual ID of the user: