[stiray]

> using Facebook and entering into a contract with Facebook is optional

It just doesn't work this way (please search for implications bellow, it was nuked during the USA daylight by people who think that lowering the visibility will make GDPR go away):

EDPS Opinion 4/2017 on the Proposal for a Directive on certain aspects concerning contracts for the supply of digital content, 14 March 2017, p. 7.

"There might well be a market for personal data, just like there is, tragically, a market for live human organs, but that does not mean that we can or should give the market the blessing of legislation. One cannot monetize and subject a fundamental right to a simple commercial transaction, even if it is the individual concerned by the data who is a party to the transaction."

Bottom line, you cant discriminate who can use the service based on giving you allowance to use personal data, if it is free for people who give you consent, then it must also be free for people who don't give you the consent.