Zuckerberg: The end goal isn't to sell the company or IPO
news.cnet.com
news.cnet.com
If you're a VC-backed company, you usually have a maximum of 7 years to make an exit. The clock usually runs out much faster since the company is either acquired long before then or it slumps and VCs take what's left of their money and put it into something else. As for IPO market... it's dead for majority of companies until SOX is overturned.
It seems reasonable that Zuckerberg is telling the truth. He will get rich no matter what, and the desire to Do Something Great is an almost overpowering driving force for some people.