You can try using this:
<?php
$title = get_the_title( $post_id );
$url = urlencode( get_permalink( $post_id ) );
?>
<a class="fb" target="_blank" href="http://www.facebook.com/sharer.php?s=100&pFacebook share button help=<?php echo $title; ?>&p[url]=<?php echo $url; ?>">Share</a>