On the page
http://developers.facebook.com/docs/reference/fbml/share-button_%28XFBML%29
It talks about the Share Button, and states that it is best to use the Like Button with Open Graph API.
It is not so entirely sure whether using the Share Button is supported by the new Facebook Javacscript SDK. The Button should be supported by the old Javacscript SDK, but not sure about the new one. Does anybody know for sure?
(the old JS SDK uses REST API, the new JS SDK uses the Graph API)