public void SendMessage(string message, FaceBookUser user)
 {
     foreach (var u in usersList)
     {
         // message should not be received by the user sending it
         if (u != user)
         {
             u.Receive(message);
         }
     }
 }
 public void RegisterUser(FaceBookUser user)
 {
     usersList.Add(user);
 }