Here is an article that shows how easy it is to build profiles of people using web sites to determine whether they have visited or not. By typing in your e-mail address and an incorrect password, many sites will volunteer the fact that you are a subscriber/member/customer.

I am sure there are companies or at least people that will use this technique to build profiles of people. It is currently a time consuming process that can be automated, making it easy to determine the likes, dislikes, political position, favorite pastimes, news sites and more via just a bit of snooping.