I had this activated on a site and we were having realy slow load times. Particularly before any content started to load. The browser would say "Waiting for domain.com...." for sometimes as much as 5-8 entire seconds! Once the site would start to load, then it loaded quickly. We looked in Firebug and saw that it was trying to connect to Hupso.com 1st! I disabled the plugin and instantly the site was much, much faster.
This makes me conflicted because I like this plugin. It doesn't load all the garbage that some Facebook button plugins do. On the other hand, I really don't like it when plugins "dial home". I especially don't like it when they do it on every single page load (front end or WP Admin) and slow the site down! That's just not cool.