In what way are they unaccountable to anyone?

Their wealth is tied up in stock whose value is tied to the perception, aka the accountability, of the general public. Not being able to personally destroy someone's wealth because you don't like what they're doing is different from being unaccountable. If tomorrow Zuck released an AI model or FB feature that was deeply unpopular, his ventures and personal wealth would dwindle according to the market's reaction. That's accountability. I'm not even a fan of Zuck... he's a slimy weasel who changes his tune to whoever is in power. But public perception directly affects his decision making.