Perhaps they figured that there was no downside to going public, if they had to disclose their finances anyways? If Zuck has 56% of the voting power, and other former and current Facebook employees have significant shares as well, then it's quite possible that even with the IPO, Wall Street won't have much sway in the company's dealings regardless.

(I am personally a big fan of Costco for rebuffing Wall Street's cries for higher prices and lower worker compensation and benefits)