I'm not sure though if that has to be the case though. Some contract types terminate automatically, question is if that is only legally possible for those special cases or if any contract can be written that way. Similarly if data deletion clauses could be challenged or not - being party to a contract that says all data has to be wiped if person X dies doesn't help you with gaining access to that data. That of course isn't part of the current social media user agreements, but could very well be added now as an option.

Then again, it‘s doubtful if a minor can enter into such an agreement.

Practically speaking, I think there are so many tangents to go off here that basically the courts could decide whatever they consider „right“, and that‘s probably in favor of the parents.

True, that's an interesting question. I'd guess it might follow the rules for writing a will, so you'd have to be 16, and even then a court might override it for the parents.