Zuck could take the rest of his life off work. And when he's an old man he will wish that he had. But in the mean time the share holders can benefit from his keen.
How do you know he wouldn't regret stopping work now when he's an old man? You have absolutely no idea. But we can take an educated guess by the motivations and choices he's shown in the past. Which were, to keep working and building his company. Because he probably enjoys it.
My grandfather grew up dirt poor. He built a small real estate business on his own, and continues to come to work every day and he's in his 90's. If he was forced to "take the rest of his life off" not just 20 years ago, but even now, he would without question be less happy as result. He enjoys working. Many people do. Especially those who are founders of businesses.
Many rich business people completely quit business and instead do philanthropy, case in point Bill Gates... Zuck's role model? Facebook is not something positive in the lens of history. It's just a big money maker. Maybe it will still be relevant in 20 years, but eventually it will be seen for what it is... as if it's not already. And all that will be left is the money, of which Zuck has more than enough to do much more worthwhile things with his life. Like perhaps philanthropy. Maybe he can really solve some problems before he dies in 40-50 or so years. Of which maybe the 10-20 next ones will be his prime.