The list of requirements sounds really fancy and uses big words but it's actually very simple.
* Agreement (Offer and Acceptance)
One party offers and the other party accepts. As they both signed this one is simple.
This is code word for money. Both parties must get something out of the deal, if only one party benefits it's a gift not a contract. In this instance Zuckerberg got paid and the other guy got ownership.
* Legal purpose
* Legality of form
* Intention to create legal relations
All of these are broadly the same thing, both parties must have intended it to be a contract. In this example the document has contract written across the top, check.
* Capacity to contract
* Consent to contract
* Vitiating factors: Mistates, undue influence, misrepresentation, duress
This are all unusual circumstance clauses. If you are mentally disabled, five years old or currently have a blowtorch being applied to your testicles you can't sign a valid contract. It doesn't seem like any of these apply or are being cited.
In general common sense goes a long way with contracts. If you intend to make a contract, both agree and both benefit you have a contract.
I am not a lawyer but have studied some law at a university level, primarily contracts. If you take legal advice from people on slashdot even if they claim to be a lawyer you are an idiot.