Facebook launched a new registration tool that gives website owners the ability to offer quick, easy and social options for sign-up. "By minimizing the friction associated with signing up for a new account and making it easy for people to bring their friends with them, we've seen that people are more likely to complete the sign up process, stay on sites longer, share more content, and come back more often. In beta tests with FriendFeed, Facebook sign ups increased 300%," stated Facebook.
When a user registers for the site with their Facebook account (similar to Facebook Login), the site is given permission to access the user's basic information on Facebook in addition to the data presented in the form, and in turn can provide the user with a personalized and social experience.
To integrate the registration tool, simply add the iframe or XFBML to your site:
<iframe src="http://www.facebook.com/plugins/registration.php? client_id=113869198637480& redirect_uri=http%3A%2F%2Fdevelopers.facebook.com%2Ftools%2Fecho%2F& fields=name,birthday,gender,location,email" scrolling="auto" frameborder="no" style="border:none" allowtransparency="true" width="100%" height="310px"> </iframe>
<fb:registration fields="name,birthday,gender,location,email" redirect-uri="http://developers.facebook.com/tools/echo/"> </fb:registration>
More Info: Documentation