I can’t find the word ‘unbiased’ in the GDPR. All it asks for is consent:
Processing shall be lawful only if and to the extent that at least one of the following applies:
a) the data subject has given consent to the processing of his or her personal data for one or more specific purposes;
In the case of facebook, the user gives consent for the purpose of being served targeted advertising in exchange for the provided service.
[Edit:] Found something:
When assessing whether consent is freely given, utmost account shall be taken of whether, […] the provision of a service, is conditional on consent to the processing of personal data that is not necessary for the performance of that contract. Article 7, paragraph 4 GDPR
So the question of whether the pay-or-consent model is legal hinges upon the question of whether payment (in any form) is “necessary for the performance of that contract“.