Hi,
How would I know? This a decision of "AddThis.com".
We have used their "share buttons" service. If you would like; you can try
sharethis.com (or google it and find another service). Basically all of them work the same way. Once you generate your HTML from sharethis.com (or any other service) copy-paste them to product details page. The product details page you need to make edits is here:
ROOT/templates/<yourjoomlatemlatefolder>/html/com_virtuemart/productdetails/default.php
Cheers.