Okay.
The fact that I prefer a type of book or a sports site, for instance, is that not a personal preference that becomes private information? Unbeknownst to the user, their use of the browser is recorded and assumptions are made about their behaviour.
If I understand this correctly, you turn to the private market and tell ad buyers that you have targets for them. Nice to provide a free service, but it does not pay the grocery bill at the end of the month.