java smack facebook获取看不见的消息

Question

我想收到离线（看不见）脸书的消息。但是我收到了这个错误。 facebook不支持此功能吗？

org.jivesoftware.smackx.OfflineMessageManager.getMessages（OfflineMessageManager.java:210）中未实现的功能（501）     在ktg_msn.test.main（test.java:58）

我的代码

 try {
     ConnectionConfiguration config = new ConnectionConfiguration("chat.facebook.com", 5222, "chat.facebook.com");
     config.setSendPresence(false);
     XMPPConnection connection = new XMPPConnection(config);
     connection.connect();
     connection.login("userrr", "passs");


     OfflineMessageManager offlineManager = new OfflineMessageManager(connection);

     System.out.println(offlineManager.getMessageCount());

     Iterator<Message> it = offlineManager.getMessages();
     while (it.hasNext()) {
         Message m = it.next();
         String fromName = m.getFrom();
         Date currentDate = new Date();
         String text = m.getBody() + " (offline)";
         System.out.println(text);
     }    
 } catch (XMPPException ex) {
     Logger.getLogger(test.class.getName()).log(Level.SEVERE, null, ex);
 }