code: function(facebookAccessToken, options) {
          options = options || {};
          options['data'] = options['data'] || {};
          _.extend(options['data'], {
            "fb_at" : facebookAccessToken,
            "token_type" : "mac"
          });
  
          (this.sync || Backbone.sync).call(this, "linkUserWithFacebook", this, options);
        }
docs: Use after a user has logged in with a regular user account and you want to add Facebook to their account