IMPORTANT: This function has been obsolete and will only work with the legacy GameMaker: Studio 1.4 extension.
With this function you can poll Facebook for the user id of the currently logged in user. This will be returned as a string, with a format similar to "07778915123" which can then be used with the other Facebook functions.
NOTE: The user must be logged in to get the ID.
fb_id = facebook_user_id();
The above code gets the user ID of the currently logged in user and stores it as a string in the variable "fb_id".