Can I use the Facebook commenting System with RapidBlog?

In a word yes:

To set up face book comments go to the 'header/footer' section and enter the following code in your interblog settings

 <?php 
 $postid=$myEntry['POSTID'];
 global $urlToThisPage;  
 $blogPostPermaLink="$urlToThisPage?id=".$myEntry['POSTID'];
 print "<fb:comments xid='$postid' href='$blogPostPermaLink'></fb:comments>";
 ?>

Set Interblog to 'insert after 0 posts and then ever 1 post' and check the evaluate PHP code button.

Then in the 'footer' section add the following code (note that this code is the same as you would add to enable facebook like button -- you only need to add it once)

<div id="fb-root"></div>
<script>
  window.fbAsyncInit = function() {
    FB.init({appId: 'your app id', status: true, cookie: true,
             xfbml: true});
  };
  (function() {
    var e = document.createElement('script'); e.async = true;
    e.src = document.location.protocol +
      '//connect.facebook.net/en_US/all.js';
        document.getElementById('fb-root').appendChild(e);
  }());
</script>

Make sure both of the above are set to 'ignore formatting' (pink) by selecting them and hitting cmd-.

You can customize the color scheme of the comments by adding the 'colorscheme' parameter and the 'width' parameter. So if you want to have it be a 'dark on light' and wider try something like this

print "<fb:comments xid='$postid' href='$blogPostPermaLink'  colorscheme='dark' width='670' ></fb:comments>";

Now you should be good to go! Be aware that your readers have to be logged into facebook to post comments.