Why we need to add <div id="fb-root"></div>

It's the place holder for the Facebook javascript script to attach elements to the DOM. Without this when the referenced Facebook script is ran it has nowhere to attach elements.

You can see fb-root gets appended to as part of the initialisation.

<script type="text/javascript">
      window.fbAsyncInit = function() {
        FB.init({appId: 'xxxxxx', status: true, cookie: true,
                 xfbml: true});
      };

      (function() {
        var e = document.createElement('script'); e.async = true;
        e.src = document.location.protocol +
          '//connect.facebook.net/en_US/all.js';
        document.getElementById('fb-root').appendChild(e);
      }());
    </script>

UPDATE: Facebook no longer requires that you include <div id="fb-root"></div> in your HTML.

You can now remove it. The Facebook Javascript SDK creates it on its own, appending it to the BODY tag. No warnings are shown in the console either, as it did before.

The Facebook documentation has also been updated, no longer showing the <div id="fb-root"></div> requirement.

Old documentation for version 1.0 (shows <div id="fb-root"></div>): https://developers.facebook.com/docs/javascript/quickstart/v1.0

Current documentation for version 2.5 (no longer shows <div id="fb-root"></div>): https://developers.facebook.com/docs/javascript/quickstart/v2.5