Share This works in HTML,frames and uses javascript.
You would need an external interface to use it. I am not sure we are on the same page when you say live in the swf. You may have to bind it to some object or the sorts to achieve it.
Did you check Barklund.org for their working on ShareThis and ExternalInterface ?
They have broken down the procedure as follows
First, simply go to sharethis.com
publisher section and customize your
widget. Instead of placing the widget
code snippet where you want the
button, put it in the head section
of your website.
<script type="text/javascript" src="http://w.sharethis.com/button/sharethis.js#publisher=636e055b-a4a2-4f9c-872c-b7aa9a701bb0&amp;type=website&amp;send_services=email&amp;post_services=facebook%2Clinkedin%2Cmyspace%2Cdigg%2Cdelicious%2Ctwitter%2Creddit%2Ctechnorati%2Cwordpress%2Cblogger%2Cgoogle_bmarks%2Cwindows_live"></script>
Then create a little javascript
function like:
function share(url, title) {
var s = SHARETHIS.addEntry({
url: url,
title: title
}, {button:false,popup:true});
s.popup()
return false;
}
Finally, simple call this function
from Flash using
ExternalInterface:
var url:String = "http://www.barklund.org/blog/2009/05/06/using-sharethis-with-flash/";
var title:String = "Barklund.org - Using ShareThis with Flash";
import flash.external.ExternalInterface;
ExternalInterface.call("share", url, title);
There are also some comments from the author saying there were problems due to the change in the API but the example worked well for me. If it does not you can check out there Using AddThis with Flash implementation