Consult a lawyer for advice on the legal implications of being directed to work on projects that either violate the law or your own ethical principles. The lawyer can help you draft an employment contract that protects you from retaliation if you object. If Facebook refuses to sign the contract, walk away.
Someone should make a museum of bizarre advice found on HN.
No normal company is going to sign _any_ contract provided by a prospective full-time employee (except perhaps if you are a sought after celebrity being hired at a VP level or above), so it would just be a waste of time and money for someone to take your advice.
Even if the hiring manager personally wanted to, there is no process for doing this. They don't have lawyers standing by to review such contracts. It would probably be hard to even find out who would have the authority to sign such a contract.
Further, retaliating against whistle-blowers is already illegal, as is ordering employees to break laws, so I don't know what additional protection you imagine you would get from such a contract.
