Plugin Name: Facebook-share
Plugin Description: Add a Facebook Share Button to each post to allow your readers to share your post with their Facebook friends. 
Plugin URL: http://wiki.developers.facebook.com/index.php/Fb:share-button
Author: Dog Lover
Author URL: http://intensedebate.com/people/doglover2688
Date Added: September 15th, 2009

Code:
function add_fb_share() {
var fb_link = document.createElement('a');
fb_link.href = 'http://www.facebook.com/share.php?u=' + window.location.href;
fb_link.className = 'fb_share_button';
fb_link.onclick = 'return fbs_click()';
fb_link.target = '_blank';
fb_link.style.textDecoration = 'none';
fb_link.innerHTML = 'Share';
fb_link.style.cssFloat = 'right';
document.getElementById('idc-container').insertBefore(fb_link, document.getElementById('idc-container').firstChild);
}

function fbs_click() {
u=location.href;
t=document.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;
}
id_add_action( 'idcomments_func_load', add_fb_share);

CSS:
#idc-container-parent #idc-container .fb_share_button { display: -moz-inline-block; display:inline-block; padding:1px 20px 0 5px !important; height:15px; border:1px solid #d8dfea !important; background:url(http://b.static.ak.fbcdn.net/rsrc.php/z39E0/hash/ya8q506x.gif) no-repeat top right; } 
#idc-container-parent #idc-container .fb_share_button:hover { color:#fff; border-color:#295582; background:#3b5998 url(http://b.static.ak.fbcdn.net/rsrc.php/z39E0/hash/ya8q506x.gif) no-repeat top right; text-decoration:none; }