All it takes is for "them" to spend a little time data mining and putting the pieces together. Lots of things are already in place, which is why when you look at products on the internet or purchase with your credit card at a store, you receive advertisements of like products in the mail and on your computer screen.
Not sure if you've ever used Quicken, but some do in order to budget money/track expenses. Lets say you go to Walmart and spend $300 and buy something from every aisle and then pay with your credit/debit card. If you have Quicken, you can log in as soon as you get home and it will show you to the penny what categories you spent money on: health/beauty, entertainment, food, clothes, etc. Imagine what the government and large corporations doing R&D for advertising has.............especially if you are linked into social media like FaceBook, MySpace, Tritter, etc.
Point being, if they want to know they will.
I agree