What really devastated me was when I found out that even if you try to keep yourself private, you'll get exposed by your friends: Just by tracking that you are

What really devastated me was when I found out that even if you try to keep yourself private, you'll get exposed by your friends: Just by tracking that you are regularly close to others, for example meeting for a camping trip, you suddenly get camping advertisements - probably because your friends searched for something or booked the trip via Gmail or whatever. I don't really know how it works, but it's awfully scary.