>> he would have to liquidate some stock [...] which would have the effect of depressing the FB stock price
Depressing the stock is not the main problem of that. Insider trading is (he cannot liquidate FB stock on short notice as he has material undisclosed information pertaining to it).
He might elect to use a 10b5-1 plan (which basically empowers the broker to make the sell decision autonomously in the future based on a target price), but if you want that cash tomorrow, borrowing against the stock is more appropriate (see for example http://www.businessinsider.com/larry-ellison-has-a-10b-credi... ).
Depressing the stock is not the main problem of that. Insider trading is (he cannot liquidate FB stock on short notice as he has material undisclosed information pertaining to it).
He might elect to use a 10b5-1 plan (which basically empowers the broker to make the sell decision autonomously in the future based on a target price), but if you want that cash tomorrow, borrowing against the stock is more appropriate (see for example http://www.businessinsider.com/larry-ellison-has-a-10b-credi... ).