Can't show some websites in iframe tag

I am trying to develop a page in which I can show more than 3 website at a time, as below:

<ul>
   <li>
      <iframe src="http://www.facebook.com/" /><p> iframe is not supported</p>
   </li>
   <li>
      <iframe src="http://www.yahoo.com/"></iframe>
   </li>
   <li>
      <iframe src="http://www.google.co.in"></iframe>
   </li>
</ul>

The problem is that it shows yahoo.com and google.co.in, but does not display Facebook in the iframe.