I see the ShareThis plugin on tons of WordPress blogs — and with reason, because the presentation is quite nice. You don’t have to take up a chunk of the screen listing all of the different social networks (and they keep multiplying…) or offering the option to email this post to a friend; instead you have a simple link that shows all of the useful detail in a little popup.
ShareThis was so popular, in fact, that a business has sprouted up around it — and current versions of the plugin are tightly bound into the ShareThis.com website. They collect data every time anyone even clicks the link to open the ShareThis window, the social networks links all redirect through the ShareThis.com servers, and all that data (associated with your website) is there for detailed reporting if you register with them. They now also encourage your visitors to sign up. I’m sure it has its uses, and they’re supported by advertising revenue, so they want lots of people to sign up and I imagine they use all that data in conjunction with showing those ads.