What? This is completely wrong. The owners don't completely cash out at the IPO. I don't know exactly how much Zuck ended up selling at the IPO, but after some quick googling I found an article from a couple of weeks ago stating

Zuckerberg, who holds a total of 533.8 million shares, would sell 30.2 million shares in the IPO — garnering about $1.05 billion in cash at the high end of the proposed price range of $28- $35 for the sale of shares.

So in otherwords Zuckerberg is worth $1 billion in cash and has 534 million shares of FB. If FB goes to zero before he sells the rest of his stock on the market, his net worth drops by something like $19 billion.