So if he some 5,000,000shares and pocketed $900,000,000.....he still owns $23+billion in the company. It’s not a meaningful decision to sell shares. Not only that, he couldn’t sell those yesterday, he can only do it during certain times between earnings and news announcements. I take this as Zuckerberg just diversifying. It is only ~4% of his holdings.
You’re missing the point. WHY did the stock drop? Read the second link.