Via Bruce Schneier come news of an important plugin
ShareMeNot is a Firefox add-on for preventing tracking from third-party buttons (like the Facebook “Like” button or the Google “+1” button) until the user actually chooses to interact with them. That is, ShareMeNot doesn’t disable/remove these buttons completely. Rather, it allows them to render on the page, but prevents the cookies from being sent until the user actually clicks on them, at which point ShareMeNot releases the cookies and the user gets the desired behavior (i.e., they can Like or +1 the page).
The add-on is also important as it highlights the fact that information is being shared even when the button is not clicked.