It's kind of like facebook authentication or something social nowadays.
This page was helpful. Maybe it's easier than you think? http://d.hatena.ne.jp/vividcode/20111025/1319547289
Note: If you don't register your site URL on the facebook developer site, there will be an error when users return after authentication http://www.nandani.sakura.ne.jp/web_all/php/2319/
Also, information on a god site called dot-install.
http://dotinstall.com/lessons/fb_connect_php_v2/8703