channelUrl and stale pages, how to avoid

Question

I am using the FB Javascript SDK in a website and it seems that the channel.php file is forcing my content to not refresh. In the channel.php file I have the following:

<?php
 $cache_expire = 60*60*24*365;
 header("Pragma: public");
 header("Cache-Control: maxage=".$cache_expire);
 header('Expires: ' . gmdate('D, d M Y H:i:s', time()+$cache_expire) . ' GMT');
?> 
<script type="text/javascript" src="//connect.facebook.net/en_US/all.js"></script>

Each page of the website which requires use of the SDK has the following included directly beneath the opening tag:

<div id="fb-root"></div>
<script type="text/javascript">
  window.fbAsyncInit = function() {
    FB.init({
      appId  : '<?php echo $cfg['fb_app_id']; ?>',
      status : true, // check login status
      cookie : true, // enable cookies to allow the server to access the session
      xfbml  : true,  // parse XFBML
      channelUrl  : '<?php echo $cfg['http_prefix'].$cfg['domain_name'].'/'; ?>channel.php', // Custom Channel URL
      oauth : true //enables OAuth 2.0
    });
  };

  (function() {
    var e = document.createElement('script');
    e.src = document.location.protocol + '//connect.facebook.net/en_US/all.js';
    e.async = true;
    document.getElementById('fb-root').appendChild(e);
  }());
(function(d, s, id) {
  var js, fjs = d.getElementsByTagName(s)[0];
  if (d.getElementById(id)) {return;}
  js = d.createElement(s); js.id = id;
  js.src = "//connect.facebook.net/en_US/all.js#appId=<?php echo $cfg['fb_app_id']; ?>&xfbml=1";
  fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));
</script>

It seems that pages which call this file "stale" in the browser cache. I was out of the office for a day and once returned the pages were still containing their old content until I did a shift-click reload on the pages. Any thoughts?