If you need to register Facebook-authenticated users as Drupal users, please take a look at: Drupal for Facebook module by Dave Cohen.
The principal difference between how this module approaches Facebook Single Sign-on integration and Drupal for Facebook one does is that DFF assumes you want to use Facebook as yet-another authentication and authorization mechanism (in addition to DB and OpenID), whereas this module assumes you do not want to authorize Facebook-authenticated users to anything in Drupal, you just want to grab the user's Facebook authentication data to use it for display (e.g. in comments and such things).
This module also exposes Facebook user data to Drupal's theming layer and third-party modules so you can use it wherever and however you need. Facebook Authentication module will soon provide optional default integration with Comments (which can be disabled).
To use Facebook authentication, you need to register your website as a Facebook application by visiting: Facebook Application Registration page
Anywhere in your theme use following variables:
Module code can access the same variables as an associative array by calling: fbauth_get_data() function.