By default, this extension will block: Google "plus one", and "buzz" buttons, Facebook "like" and "connect" buttons, "tweet this" buttons, share on Linked In button, Stumbleupon buttons, and the "Add This" button.
Currently the extension will block these widgets only if they are usually their official API endpoints.
Users can toggle any of these on or off in the preferences page. For example, this extension probably breaks facebook.com. But if you're installing it, there's a good chance that's a feature. If that bums you out, you can turn off Facebook blocking.