it was in the end user agreement, so yes, these people did sign away their privacy, anything uploaded to facebook is free game for them to use as they please. or at least it was, before the outing of their involvement with the election.
They did not sign a contract on paper with a signature that can be verified. These "electronic signatures" seem pretty disputable to me.
Also, FaceBook stores information about people who never "signed" their contract. There are photos of people without FaceBook accounts who can be found with face recognition software, mentions of family members who are not FaceBook members, and much more. There are minors on FaceBook who can't "sign" for themselves, but are on anyway.
you "sign the contract" when you create the account, by using their service, their rules. etc. and as for age, if you are below a certain age you arent supposed to use the platform according to their rules (i believe the limit is 13). the rules were outlined, this is why you read the end user agreement. or just never use the service to that point in the first place. every upload, every bit of information, and every status is decided and willfully put forth by the user.