We added Facebook comment area to this blog using comment plugin (blog parts) provided by
http://developers.facebook.com/docs/reference/plugins/comments/.
But this code supports the TOP URL only, so even if visitors comment the post, the target links are always the top url.
How to link to each post via facebook comment plugin is like this. (The case of WordPress blog)
echo ‘<br/><div id=”fb-root”></div><script src=”http://connect.facebook.net/en_US/all.js#appId=APP_ID&xfbml=1″></script><fb:comments href=”‘;the_permalink();echo ‘” num_posts=”2″ width=”500″></fb:comments>’;
the_permalink() is WordPress function to give each post link, so you need to replace the url with it.
*CAUTION: the_permalink() outputs the link by itself , not link strings, so you have to split the code into some ‘echo’s.
(echo ‘<fb:comments…’ + the_permalink() + ‘/fb:comments>’ will not work.)
Other blogs that support post link API or variables will do in the same steps, I think.

Recent Comments