/// <summary>
 /// Retrieves an instance of ClaimsIdentity based on user Facebook id
 /// </summary>
 /// <param name="user">The latest information about the user that FB provides</param>
 /// <returns></returns>
 protected abstract ClaimsIdentity Retrieve(FBUser user);
 /// <summary>
 /// Retrieves an instance of ClaimsIdentity based on user Facebook id
 /// </summary>
 /// <param name="user">The latest information about the user that FB provides</param>
 /// <returns></returns>
 protected abstract ClaimsIdentity Retrieve(FBUser user);