You're absolutely correct... However it will be very interesting to see how this doesn't violate the GDPR... recital 42 says:
“Consent should not be regarded as freely given if the data subject has no genuine or free choice or is unable to refuse or withdraw consent without detriment.”
Link with more details: https://gdpr.eu/gdpr-consent-requirements/
Withdrawing consent in this case causes the detriment of having to either pay or lose access to the service... So this clearly isn't "freely given" consent.
The GDPR does not in any way disallow Facebook from running ads, regardless of the users consent. But if the user doesn't consent, Facebook can't run targeted ads on the user.