What is Mark Zuckerberg's Net Worth?

Facebook hit another new low today of $26.57, which is down 30% from its IPO price of $38.  While investors who got in on the IPO and haven't gotten out are nursing wounds, Mark Zuckerberg is quite possibly feeling some pain of his own.  His paper net worth has fallen by $6 Billion since the IPO, from $19B to $13B.  He owns just over 500m shares, so every dollar price drop represents half a billion dollars in losses for him.  Poor guy...