Claim a property interest in your personal information
If you're wondering what personal information the ad networks and other third parties are collecting based on your online activities, don't expect the collectors to be very forthcoming. For one thing, they may not know how the parties they sell the information to will use it. Who knows how many times the information is rehashed and resold?
Even though the services claim to anonymize your personal information, doing so doesn't reduce the value of the raw data to the person from whom it was collected. You could posit that the personal information had no inherent value until it was collected and aggregated, etc. This is the opinion of most courts to date, as I point out in "Reclaim your personal information." Therefore, the person lost nothing of value.
Your personal information still belongs to you after it has been anonymized. If the collector realizes value from the information, you have a right to the fair value of your information, despite the collector's claims that the data has been "depersonalized."