I heard of yet another way to do the share button, which is on
http://www.facebook.com/share/
It include this Javascript file:
<script src="http://static.ak.fbcdn.net/connect.php/js/FB.Share" type="text/javascript"></script>
Is it compatible with the old and new Facebook Javascript SDK too?
The share functionality is part of the old Javascript SDK. The share functionality is being deprecated and replaced with the "like" functionality. I would not recommend using a share on a site that also uses the new Javascript SDK.
From facebook on http://developers.facebook.com/docs/share:
"We don't recommend the Share button for new developers. If you aren't already using the Share button, we recommend you use the Like button and Open Graph protocol instead of Share for sharing pages from your website. The Like button is simpler to user and is the recommended solution moving forward."