gistfile1.js

document.observe("dom:loaded", function(loaded) {
  $("login_via_facebook_connect").observe("click", function(clicked) {
    FB_RequireFeatures(["Connect"], function() {
      FB.init("FACEBOOK_API_KEY", "path/to/fb_crossdomain_communications_channel.html", {
        ifUserConnected: function(uid) {
          // SessionController#create will get the Facebook session from the 
          // cookie set by Facebook on the Cross Domain Communication Channel
          clicked.element().up("form").submit();
        }
      });
      FB.ensureInit(function() {
        FB.Connect.requireSession();
      });
    });
    clicked.stop();
  });
});