getUserProfile(userId) {
    const url = `https://graph.facebook.com/${this.graphApiVersion}/${userId}?fields=first_name,last_name,profile_pic,locale,timezone,gender&access_token=${this.accessToken}`;
    return fetch(url)
      .then(res => res.json())
      .catch(err => console.log(`Error getting user profile: ${err}`));
  }
Returns a Promise that contains the user's profile information.
@param {String} userId
@returns {Promise}