In a response in php we can do this:
<div class="fb-comments" data-href="<?php echo $url; ?>" data-width="687" data-numposts="7" data-colorscheme="light"></div>
<?php
$url = (!empty($_SERVER['HTTPS'])) ? 'https://'.$_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI'] : 'http://'.$_SERVER['SERVER_NAME'].$_SERVER['REQUEST_URI'];
?>
However, I use node.js and jade template engine, with jade template engine we can put in the "master page" a "block" and inside the block the content that will appear on all other pages, as it could do to have dynamically each url of their pages, in Javascript, without having to put a comment box with their respective url on each page of the site?