code: function _getPersistantFacebookToken(){
      return facebook.query()
              .get('/oauth/access_token?' +
                  'grant_type=fb_exchange_token&' +
                  'client_id=' + appId + '&' +
                  'client_secret=' + secret + '&' +
                  'fb_exchange_token=' + this.options.code)
              .request()
              .then(function (res) {
                  // Bad tokens result in a 200 with an error object
                  if (res[1].error) {
                      throw new Error(res[1].error.message);
                  }
  
                  this.creds = querystring.parse(res[1]);
              }.bind(this));
  }
docs: Function will accept the passed in authentication token from FB and exchange it for a permenant token. This token will be saved to the user's identities profile.