1.Log in to your dashboard--> layout- -> Edit HTML
2.Click on "Expand Widget Templates"
3.Scroll down to where you see this:
<p><data:post.body/></p>
4.Copy below code and paste it immediately after the line <p><data:post.body/></p> .
<a expr:href='"http://www.facebook.com/sharer.php?&u=" + data:post.url' target='_blank'>Share on Facebook</a>
5.Click on "Save Templates" and now you are done.