‘Add This’ gives you an easy way to add social media buttons to your site in a “consistent” manner. No need to add a bazillion js includes for every site you want to support.
The catch? They track your users. You can’t opt-out your entire site.
It sure makes those buttons easy to add, but yeah that’s the price: someone will be tracking your users on your site. Incidentally, twitter is also doing this now with its tweet button, but you can opt-out on a site-level (which is good!).