The New York Times has a brief piece on how ebooks allow reading habits to be tracked. That example might seem quite benign, although I do wonder how much I or anyone else can rely on their claims of anonymity. However, it is not too hard to imagine this kind of technology being used by totalitarian regimes to effectively track how people think in a very 1984 way. Indeed it would not surprise me if GCHQ and Barack Obama's NSA do precisely that now, possibly retrospectively with all the "metadata" they have collected.
Immanuel Kant argued that even the most totalitarian regime could not look inside your head. Those days seem to be at an end.