Steps:

• 1. Install Jumi package or Custom code module on your website. These two modules both allow you to put PHP code in modules.

• 2. Go to Facebook comments module page or Facebook Like buttom module page, click "GET CODE". Copy sample code.
  You will get something like that for Like button:
  

  <div id='fb-root'></div><script src='http://connect.facebook.net/
  en_US/all.js#appId=241285629235863&amp;xfbml=1'></script>
  <fb:like href='example.com' send='true' width='450' 
  show_faces='true' action='like' font=''></fb:like>

  ...or code will look like this for Comments module:

   <div id='fb-root'></div><script src='http://connect.facebook.net/
  en_US/all.js#xfbml=1'></script><fb:comments href='example.com'
  num_posts='2' width='500'></fb:comments>

• 3. Paste that code to Jumi or Custom code module content.
  
  

• 4. Insert following code between quotes in href='' part of Facebook code.
  

  <?php echo "http://www.yourdomain.com".$_SERVER['REQUEST_URI']; ?>

• 5. Publish Jumi or Custom code module at the bottom position of your article and you are done!