Try using this, it should make it fall in line with tag box, who has read etc
Code:
<!-- FB Comments -->
<h2 class="blockhead" style="padding-top:5px;"><span class="optiontitle">Facebook Comments</span></h2>
<div id="thread_tags_list" class="thread_info_block blockbody formcontrols floatcontainer">
<div class="blockrow floatcontainer">
<div id='tag_list_cell'>
<div id="fb-root"></div><script src="http://connect.facebook.net/en_US/all.js#appId=APP_ID&xfbml=1"></script><fb:comments expr:xid="APP_ID" simple="1" num_posts="10" width="500" publish_feed="true"></fb:comments><br/>
</div>
</div>
</div>
<img src="clear.gif" height="10" /><br />
<!-- /FB Comments -->