Funny, I'm pretty sure I could figure out who you are (or at least your likely demographic) without even a truly invasive search based on simple information gleaned from monitoring your data usage (to say nothing of the fact that your ISP has plenty of personally identifying information on file with each contract).
Your location, IP address, applications that frequently connect to the net, port usage, browser information, and so on can always be linked back to you. Even if you were wrongly identified, we're talking demographics here so no harm, no foul. No advertiser cares if it's the kid with the $50 electric guitar or the $80 electric guitar. Both kids play cheap electric guitars and would probably be interested in lessons...
Or a new guitar...
Or a guitar-heavy new album...
And that's all information that can be stored, transmitted anywhere, sold, subpoenaed at any time. Just the fact that I could mail mortgage/real estate information based on the financial institutions you frequent and the addresses of your most frequent locations, credit offers based on your shopping and financial history, and bogus 3 day, 2 night getaway vouchers every time you look up Vegas should be enough to demonstrate just how easily that information can be mined and resold.