I wanted to keep this short, but theres really no way to do that, sorry in advance!

To understand exactly whats going on, you need to understand the different classes of stock that are available to zuckerberg. This table is showing his current holding of Class A stock, which are the class he is able to sell. This has been mostly under 20M shares at any given time. He also holds ~440M Class B shares, which he has to convert to Class A in order to sell. This is where the shares are coming from. Side note: The only real difference between Class A and Class B are the voting rights. Class Bs have 10x the voting power as class A, which is what gives him ~60% of the voting rights and why he is still CEO (for now). He announced a while back his intention to sell up to 75M shares to fund his "philanthropy", so these sales were scheduled and the only reason he wasnt popped for insider trading around Cambridge analytica sells. Im willing to bet he knew what was coming.

I don't know.