IPO is driven at least partly by market demand for a share of the money that a potentially huge company will pay back to investors. This is especially true for FB, which is not short of cash.
Facebook said it had plenty of money to meet its business needs, and it plans to simply save the $5 billion it plans to raise from investors.
While the best venture capitalists don’t mind waiting patiently for a big return, even if it takes a couple bad quarters to get there, the great mass of investors is far less patient.
But Zuckerberg’s unique control over the company means that investors aren’t buying something they have control over, they’re just betting Zuckerberg won’t screw it up.