On Facebook, you are the product. Advertisers are the customer.

Source: Is your data safe on Facebook? Not really – Mar. 19, 2018
Is Facebook turning over our private data to advertisers? Yes, probably.
What happens to it when it gets in their hands? Who sees it? Who interprets it? Who uses it? Who destroys it – if ever? How is it erased securely (if erased at all)?