Zuckerberg's stock a bit different: the stock represents future income flows - they have real value - whereas cryptocurrency value is a function of confidence.
Yes, the stock would go down on Zuckerberg selling, but that's because people would infer from it an estimate of a decline in future income flows. But the future income flows won't go away just because Zuckerberg sells; it's not just confidence putting a floor on the stock.
Whereas Mark Zuckerberg could exchange his shares for dollars ... which supposedly represent future tax income of the united states government ... paid in dollars.
Fiat currencies, including somewhat non-intuitively shares, dollars and every currency (including most forms of gold I might add) ultimately depend on confidence.
Yes, the stock would go down on Zuckerberg selling, but that's because people would infer from it an estimate of a decline in future income flows. But the future income flows won't go away just because Zuckerberg sells; it's not just confidence putting a floor on the stock.