code: public int auth_getUserId(String authToken)
      throws FacebookException, IOException {
      /*
       * Get the session information if we don't have it; this will populate
  	   * the user ID as well.
  	   */
      if (null == this._sessionKey)
        auth_getSession(authToken);
      return this._userId;
    }
docs: Call this function to get the user ID.
  
  @return The ID of the current session's user, or -1 if none.