I found that my FB Comments wouldn't appear on my page if I entered a full url:
<fb:comments xid="http://www.mywebsite.com/index.php" numposts="10" width="425" publish_feed="true"></fb:comments>
but appeared OK if I used a document-relative path:
<fb:comments xid="index.php" numposts="10" width="425" publish_feed="true"></fb:comments>