I also don't understand how it works. If you could up-vote just by following a link, you could easily gain a system. For example, you could submit a story that automatically up-votes itself when visited by logged-in user, without any button press (CSRF). I believe Facebook like button (and other similar gadgets) are iframes from Facebook. So Facebook controls how the button looks like and it is not possible to submit 'Like' requests from a third party domain.