Based on what the big investors (like Microsoft) have been willing to pay to own a piece of the company, Facebook now has a valuation of about $23 billion.
Obviously, when it goes ipo, it will be worth significantly more.
I personally think that Facebook will trade over $100 billion market cap a few years after it goes ipo.
That means Mark Zuckerberg, who owns about 24% of Facebook – valued at $5.5 billion based on the $23 billion market cap – will be worth $22 billion at the $100 billion market cap valuation.