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.
* Consideration
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.