Dustin Curtis:

Basically, every time you visit a site that has a follow button, a “tweet this” button, or a hovercard, Twitter is recording your behavior. It is transparently watching your movements and storing them somewhere for later use. Right now, that data will make better suggestions for accounts you might want to follow. But what other things can it be used for? The privacy implications of such behavior by a company so large are sweeping and absolute.

(Via Daring Fireball.)