very strange. I have the same problem. I developed the Facebook application and I used the function to post to the user's wall. But there is no Share link on the user's wall. It is only possible to love or comment on it.
If I use this other technique, a dialog box appears displaying the message (on the wall) or “send as message”. (I was also looking for this feature).
And it's good that you can like the resulting post on the wall, AND Shared commented.
This is the javascript code that will help you get this functionality:
<script>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;}</script><a rel="nofollow" href="http://www.facebook.com/share.php?u=<;;url>" onclick="return fbs_click()" target="_blank">Share on Facebook</a>
Taken from http://askville.amazon.com/add-Share-Facebook-button-webpage/AnswerViewer.do?requestId=1539369
Hope this helps. It helped me at least;)
source share