Now that Facebook is publicly traded, could Zuckerberg simply decide to sell off all of his shares? If he could, what effect would that have on Facebook’s overall value? Is his presence there really that big of a deal anymore?
Selling that number of shares would send the stock price down. Plus, Facebook without Zuckerberg would have a much less bright future, so it would further tank the stock.
There’s nothing stopping him from selling out at this point, though there are some obvious consequences:
- He’d lose voting rights as a shareholder. He could still be CEO, chairman of the board, etc. but it would be only so long as the other shareholders like him there. The more of your voting rights you lose, the more you have to worry about keeping other people happy. (If you had 51% of the stock, you could ignore the other shareholders entirely, since your vote would always be the majority). An implication of this is that Zuckerberg’s ownership of stock and his presence at FaceBook are not directly connected - he can own it without being present and be present without owning it.
- Such a large sale might make it hard to find buyers at a good price. The market price of a stock just records the last price at which a buyer and a seller agreed on a price. In general, there are always potential buyers who want to pay less and potential sellers who want to receive more. So if you’re motivated to sell off very large amounts of stock, you’ll probably have to accept a lower price.
- Stock transactions by key individuals in a company are reported to the SEC and made public to other investors. If you saw the founder cash out, how much confidence would you have in the company’s future growth? If he doesn’t think his own company is a good investment, why would you?
You might want to do a little research on Bill Gates and his relationship to and ownership of Microsoft. This may illustrate how a company founder can step back from running the company and one strategy for slowly selling off shares.
Agreed that he couldn’t cash out completely without tanking the stock price. Probably could only take out a few lifetimes worth of luxury living before anyone noticed. 
But I don’t think Zuckerberg pulling out would drop share price because people would think facebook would have a less bright future. Zuckerberg has not been the technical or corporate centre of the company for a long time. I think it would drop the share price because people would think, “does he know something about an upcoming calamity that he’s trying to avoid being there for?”
He will be restricted under an agreement with the underwriters of the IPO from selling any shares for at least 180 days. After that, as an insider, he will be subject to certain limitations on the volume of shares he can sell over a given period, and will be restricted from selling shares when he possesses (or would be deemed to possess) material nonpublic information about Facebook. I haven’t looked, but some of his shares might actually be contractually restricted incentive shares or unvested options as well. All this in addition to the commercial reasons for not wanting to dump all of his holdings in a short period of time, like those described by dracoi above.
He could sell his shares (subject to restrictions) and still remain CEO.
He’s delegated power, certainly, but he’s worked very closely with the delegates. He controls a majority of voting shares, and it looks like he will for some time. It’s hard to see facebook’s trajectory as anything other than an extension of the will of Mark Zuckerberg.
As Hopesperson said above, the answer is, he can’t sell shares whenever he wants to. He is prevented from doing that by law or contractual agreement.
If the Van Gogh Museum decided to go out of business and sell its entire catalog one day at auction - do you think each Van Gogh would still be worth tens of millions?
Plus, as others have pointed out… if he bailed, what do you think investors would think that implies? Future technical or legal problems coming? Big stock price drop coming? Guy with great vision no longer calling the shots? All of the above?