Are there any rights you think should supersede contracts?
That is beside the point I’m making. Facebook acknowledges the right to privacy by giving you the choice to pay for the service rather than giving up your data. In my view, this should be completely acceptable by the GDPR. No-one is forcing you to sign up to facebook, so you do have a completely free choice to (1) either not give up your data and not use facebook; or (2) not give up your data and pay for the service; or (3) give up your data and pay for the service that way.
(I’ll answer your question in a comment side-chain, just because you asked.)
Germans have the right to continued wage payments if they need to take care of family members (§616 BGB). However, that right can be voided in the employment contract.
(§618 BGB) essentially states that the work environment must be reasonably safe. This cannot be voided by contract, as is codified in (§619 BGB).
These are just instances. I do not know any general rules for the precedence of contracts over the law or vice versa.