Basically, every time you visit a site that has a follow 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.
The other implication is that every website you visit that includes “tweet this” buttons or
<iframe>-embedded tweets is enabling Twitter to track you on the web.