It's an unrealized asset. It can be included as a footnote on his personal net worth but isn't part of it until it's sold. For a founder, the red-tape involved in selling stock is significant and they must wait for the market to absorb the announcement of their intention to sell. I recall when Bill Gates sold something like $200M of his stock to pay for his lake-side mansion and that the SEC filing went out of the way to express that Gates didn't really want to sell anything but it was the only way he could finance his new mansion... in other words, there's no insider knowledge about Microsoft that prompted his selling. For Zuck to cash out he would have to announce well in advance and then say he's going to sell X percent every first trading day of the month (or whatever period he defines) whether the stock is going up or down in order for it to not be taken as an insider move.

(In fact, selling covertly or converting stock to any financial instrument covertly -- what's called "buying a collar" -- is quite illegal and will land you in jail if caught.)