Here is the code you need (don not let the HTML entities scare you - copy this exactly):
You will want to add this to or below the footer section of your template's posts includible e.g. after the third footer line.<script>
function fbs_click()
{
u=<data:post.url/>;
t=<data:post.title/>;
window.open('http://www.facebook.com/sharer.php?u='+encodeURIComponent(u)+'&t='+encodeURIComponent(t),'sharer','toolbar=0,status=0,width=626,height=436');
return false;
}
</script>
<style>
html .fb_share_link {
padding:2px 0 0 20px;
height:16px;
background:url(http://static.ak.facebook.com/images/share/facebook_share_icon.gif?43:26981) no-repeat top left;
}
</style>
<a class='fb_share_link' expr:href='"http://www.facebook.com/sharer.php?u=" + data:post.url' onclick='return fbs_click()' target='_blank'>Share on Facebook</a>