Facebook doesn't sell information, it sells ads. That's not the same thing.
The companies that try to sell those types of profiles, by IP or cookie syncing or whatever, seemed to get it wildly, wildly wrong - but the role of accuracy/auditing in the ads business is another discussion. FB seems to be buying a different sort of data than we were, since they know so much more (first/last/birthdate, etc).
Anyway, this is a problem that doesn't start or stop with FB, and the brokers are the dangerous part.
As people shy away from phone calls, and with the competition of VoIP, we gradually saw the erosion of the price per minute we could charge (roughly 40% drop over the course of 5 years).
The company tried to shift by looking at the data we had available. Millions of phone calls, and we knew exactly what kind of service was being called. Banks, airlines, airports, sex lines, restaurants, customer support, whatever. We could see when people called most, for how long, and we even knew which service inside the call centre they were talking to.
It wasn't long before the company wanted to exploit this, and they started working on the ability to play ads while people were waiting to get in touch with a service representative. Targeted ads.
Based on phone habits, we could tell whether the person calling was a stay-at-home parent, retiree, teen, young professional, etc. We could also tell their gender, in 60-70% of cases.
Because of the technology, we had access to actual phone number, even when people called with restricted identity. It's just how it works when you are registered as a "network operator".
This shift from "let's provide an amazing service" to "let's mine the crap out of everyone" definitely contributed in my leaving the company.
That's not really possible. Proof: Say Alice is your client, and her ad targets gay people (even though the ad or the advertised product itself is not necessarily related to being gay). Then using your data about sexual preference, you serve the ads only to gay people. Only gay people will see the ad, and thus only gay people will reach Alice's website. Hence Alice got the data out. QED.
Imagine the government put a policeman on every corner and they recorded the movements of every person in the neighborhood at all times. 4:36pm Mr. Jones walked to newsstand, 4:43pm Mrs Jones opens front door, looks around, closes front door. 4:45pm Mr. Smith dog starts barking. 4:51pm Unknown man shows up at Mr. and Mrs Miller's house. Mr Miller is not home. She invites him in. 5:01pm Mrs Wayne tends to her tomato garden.
Many (most?) people would feel creeped out to know they are being constantly watched and all activities logged. And yet Microsoft, Apple, Google, Facebook and a bunch of others are trying to do exactly that through your computer, your online activity and through your phone and eventually they'll probably also do it with cameras and mics