As mentioned here: http://support.addthis.com/customer/portal/questions/193945-page-layout-posting-on-facebook
I have the same problem. I try to share to Facebook...when I click the facebook icon at the addthis website (example area) you'll get a well-formed page to post to your wall. The url is http://www.facebook.com/dialog/feed?etc.
When I use it at my website the button redirects to http://www.facebook.com/sharer/sharer.php?etc.
What is the difference and how to fix it? A second reason to fix this is the possibility to use a custom title, url and description. In this way it won't work out.