Let's say he exercised options that earned him $5BB. (I don't know what the exact amount was. It was in the billions, no doubt.) So let's say maybe he owes $2BB in taxes. As for the rest of his shares and what they are "worth", those are going to fluctuate in value, based on the company's performance.
Now, let's say you are a FB shareholder. Ask yourself as a shareholder if the FB CEO has earned a $5BB salary. What has he done for you? How is your FB stock doing? OK, so let's say you think he is a genius and certainly deserves $5BB. Then ask yourself why he would want $5BB in income in _one year_. He wants it all right now. That's an awful lot of income. And a lot of tax to pay.
Why the rush to cash out?
Whatever happens to Facebook, he is set for life. Good for him, but not necessarily good for Facebook shareholders. He has little incentive to deliver.
Wall St. is going to love him, I can already tell.