The dystopian scenarios are fun to think about, but realistically speaking what happens is your data is used for market research. The vast majority of it is used to curate your advertising profile more effectively or to generate financial insights about companies based on consumer habits.
In my experience anonymization is hit or miss, but ostensibly always in place.
It gets sold to private investigators and insurers, and you shouldn't have ever Googled for "alzheimer disease symptoms" and you lose your child custody rights after a judge decides what you're looking at on YouPorn isn't sufficiently healthy.
Build a social/interaction graph and fill everything with Ads while selling the same information to intelligence agencies. No discretion. Only money. Nothing personal, it's just business. Sounds familiar? Facebook invented that already.
A lot of people focus on how this information is used by advertisers, which can be a problem, but in my opinion the bigger issue is how data brokers are starting to act like completely unregulated and sometimes highly inaccurate credit reports.
