Ever get the feeling that you're being watched? That someone is following you? That the guy sitting on the bench over there-guy in a trench coat, guy holding a large newspaper with two eye holes cut out-is actually spying on you? Or maybe it's something a little more insidious. Those Amazon ads you keep seeing. Google Predict's uncanny knack of knowing what you're searching for after typing just two letters. That Facebook getaway suggestion that you only talked to your friends about.