> when you provide Facebook an email address, they will explicitly pay a lot of databases to cross-reference your email address and tell Facebook your identity, your salary, and so on.
Just do a quick search for "DMPs" or "data management platforms."
Multi-billion dollar industry that's focused on collecting data from many different sources, consolidating, and aligning towards real individuals with a combination of deterministic data and probabilistic assumptions.
Then, they can sell access to that database to various companies, mostly in the ad-tech space.
Source: did consulting work for an ad platform in the RTB space on the DSP side, competitor to Google.
EDIT (more context / sidebar thought): this is also why Apple deserves some credit here for their moves, as they are one of the few companies with enough of a war chest to fight against these multi-BILLION dollar interests. It's the type of advantage I worry about losing if Apple has to open up different app stores on the iPhone: if a developer doesn't want to submit documentation and/or get bad publicity for lack of a privacy label, they'll just go to a different, less strict app store.
A search for prior HN discussions matching keywords 'facebook' and 'data' provides many interesting discussions and links to review, and I encourage you to take a look if you're interested to learn more. (If you're already familiar, then with apologies, I won't be engaging in discussion about sentence in this thread. It's possible my summary is imprecise or wrong in some manner; it's presented here only to support answering the question asked, and for that purpose it's enough as-is.)
Can you elaborate on this?