Modern programmatic advertising enables each user to be pimped out to whichever manipulator is willing to pay the most. In just a few dozen milliseconds, Facebook will send out your personal data to participants in the advertising system ('bidstream data'), select a winner, and serve their ad to you.
Note: Your data goes out to bidders, and someone pays Facebook for it. That sure sounds like selling your data.
This is not some farcical, imaginary backdoor to collect data and spy on people. It's happening right now. "Patternz", an Israeli spy-tech company, run an advertising service as a front to get free access to bidstream data from 87 ad exchanges and SSPs including Google, Yahoo, MoPub, AdColony, and OpenX.
They take those bidstream data, de-anonymize them, and permanently record them. They claim to have profiles on billions of users including their location history, home address, interests, information about 'people nearby', 'co-workers' and 'family members'.
See also this slide of theirs: https://pbs.twimg.com/media/F-5bA6QW8AAyfSK.jpg, entitled, "We help national security agencies detect audience patterns and user behavior using digital advertising data mining and analytics".