I'm not using the Add This code like you mentioned, but I just added a FaceBook share link to my product pages.
I wanted the FaceBook Share link to show just under the product image and description. While I'm sure it's not perfect it seems to be working for me.
Note, I have all my product images in the main 'images' folder and the root of my ZenCart store is in the folder '/shop' and not at the root of my site.
In 'templates/your_template/templates/tpl_product_info_display.php; find
PHP Code:
<?php if ($products_description != '') { ?>
<div id="productDescription" class="productGeneral biggerText"><?php echo stripslashes($products_description); ?></div>
<?php } ?>
<!--eof Product description -->
After that code, add the following code, I'll explain below.
PHP Code:
<link rel="image_src" href="<?php echo "http://" . $_SERVER['HTTP_HOST'] . "/shop/" . $products_image_medium; ?>"/>
<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><style> html .fb_share_link { padding:2px 0 0 20px; height:16px; background:url(http://b.static.ak.fbcdn.net/rsrc.php/zAB5S/hash/4273uaqa.gif) no-repeat top left; }</style><a href="http://www.facebook.com/share.php?u=<?php echo "http://" . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'] . "?" . $_SERVER['QUERY_STRING'];?>" onclick="return fbs_click()" target="_blank" class="fb_share_link">Share <strong><?php echo $products_name; ?></strong> on Facebook</a>
If your store is at the root of your site, in the <link> section (for the FB thumbnail) instead of what I used:
PHP Code:
<link rel="image_src" href="<?php echo "http://" . $_SERVER['HTTP_HOST'] . "/shop/" . $products_image_medium; ?>"/>
you should be able to use this:
PHP Code:
<link rel="image_src" href="<?php echo "http://" . $_SERVER['HTTP_HOST'] . "/" . $products_image_medium; ?>"/>
Below is a screenshot of my product page with the resulting FB window that opens. You can also see I added the product name, in bold, to the FB link text. Hope this helps.