Do Google and Facebook really sell data? From my understanding, they keep the data for ad targeting, feature/content recommendations. Like if I place an ad on Facebook I could put 18 to 35, who likes travel, etc to promote a hotel booking site as an example. Even if you do retarget with Facebook advertising, they don't even show you the names of the people. You have like a retargeting list, it tells you how many people but not much information on them.
Even third party developers get an ID number per user associated to that app only(think of it as a proxy to your real id number). It seems like Facebook and Google cares a lot more about privacy than people think. If they just sold me the data, I could run off with it but by keeping the data to themselves and letting marketers pay them to make use of it they make recurring revenue.
From my understanding, LexisNexis and other data brokers were the ones that are selling the data. Here's a 60 minutes clip on it https://www.youtube.com/watch?v=qAT_ina93NY
I don't have any relationship with LexisNexis, I never agreed to anything yet they have information on me. Another example is I was looking at my credit report, and it even lists addresses from childhood. I never had a credit card back then, why do they have information on me? At least with Google and Facebook, I gave consent and I see a benefit. You can even buy lists of people and their medications(sounds like a HIPPA violation but the data brokers know this stuff)