Your comment is excellent and the truth. By selling a person's secrets, you control them. Spending habits, political beliefs, sexual preferences, gambling/drinking problems. You are owned when you reveal this data.

Facebook track people through IP addresses, Facebook logos, partnerships, etc. So they know what you say, what you look at, when, who you talk to etc.

If the wrong people get that data (including espionage, change of management etc), it could get VERY ugly.