"In order to provide tagging service to our users, a key platform feature, we are required to maintain and process data about individuals with which Facebook, Inc. does not have a pre-existing business relationship ("Non-Users"). Failure to do so would cause substantive harm to our service, and should therefore be exempted from gathering consent from Non-Users under Article 21, Section 5 of the GDPR."
Something like that, I expect, although I'm not a lawyer either.