When you’ve worked for tech companies that had their stock drop 20+%, I’m sure it’s been in response to some relevant information that would affect profitability, such as the company’s earnings coming in low, a bad product launch, a competitor’s good news, a recession, etc. The companies have also had a fighting chance to change their future prospects. The scrambling may not have been enjoyable, but it was rational.

In Facebook’s case we aren’t talking about any new information that would affect the stock price. The dual-class nature of the stock is well-known. Individual investors may not have known, and they may have bought the stock based on wrong information. But for the most part “Zuckerberg still has supermajority voting control” isn’t an actual headline. The only new information is that we now have one datapoint about what Zuckerberg will do when he disagrees with investors. I don’t think that changes the value of the stock in any meaningful way.

The end result is that there’s no reason to believe that upset investors selling off their shares would have any long-term effect on the stock’s price: the moment the price drops, other investors will bid it back up. If Zuckerberg loses any sleep, it will be for less than a week and he wouldn’t need to make any changes (he wouldn’t have time to!).

I suspect you and I need to agree to disagree here. You seem to believe the stock market is strongly efficient. Whereas I think it is weakly efficient. We're not going to change each other's minds but I stick to my belief that you could ruin Mark Zuckerberg's day for quite a while selling off the stock. Failing that, donate money to a political candidate like Elizabeth Warren that will ruin his day for you.

You are correct that I believe the US stock market is generally strongly efficient.