<a class="addthis_button_facebook_like" fb:like:send="true"></a>
This will look like this:
If you just want the Facebook Send button, use this code:
<a class="addthis_button_facebook_send"></a>
More information about these buttons is available in our help documentation: http://0-www.addthis.com.innopac.up.ac.za/help/custom-buttons