Use Facebook Connect
Schönes Tutorial mit guten Erklärungen wie man Facebook Connect auf der eigenen Seite nutzen kann.
This tutorial will guide you to the process of integration of your web community with Facebook. This means that when a visitor comes to your site it can log in with normal credenditals if it has, but if it has not it can click on the “facebook connect” button and try to log in with its facebook account, automatically.