Mostly ad targeting (you can infer a lot of things from the global graph of contacts). Meta is an attention routing company.

How do they spend insane amount of money in targeted ads and all the ads I get are useless?

I constantly get ads to learn how to code. Ok I've been doing that professionally for over a decade and I have a real degree from a real university… why would I do some online programming course?

There are much more ads than viable targets.