title: socket.io - broadcast to certain users
body: <p>I need to build twosome chat, using websockets (socket.io + node.js).
  So, the simple example to broadcast message to all users:</p>
  
  <pre><code>socket.on('user message', function (msg) {
      socket.broadcast.emit('user message', socket.nickname, msg);
    });
  </code></pre>
  
  <p>But how can I broadcast it from certain user to certain user?</p>