What if the new owner decides to "generate revenue" by selling your personal information to the highest bidder, and then turns the site off? Reselling the information is cheaper than keeping a bunch of programmers and sysadmins employed, after all.

Could you add a clause in any (subsequent) contracts dealing with this possibility?

If this contract is valid, Zuckerberg would have known about it ever since 2003. If his business associates knew about it, would they have dealt with the situation proactively?

If he chose not to disclose the information, maybe he'll be seen as a danger to the future prosperity of the company?

If the previous contract is deemed valid, then the later contracts are usually "breached" and they can sue for damages. This means that the first investors all the way until through the latest investors have contracts signed by Zuckerberg or company executives stating the beneficial ownership. And this ownership is no longer accurate.