What are they going to do, exactly? They explicitly invested in the company knowing that Zuckerberg would retain full control.

If they can show gross negligence there may be a legal avenue, but it would be pretty hard to justify chasing potentially profitable business ventures, even if they end up failing, as being negligence. Controversial business decisions are not negligence in the eye of the law.

Sure, they can sell their interest in the company — if someone else wants to buy it — but that just moves who the investor is around. That doesn't really change anything.