The valuation is a bit back of envelope, which might be considered grossly irresponsible at your discretion — other comments in this page seem to support the share price range, I honestly don't know. But the concept is correct: market cap = total shares * share price.

What's slightly disturbing is how they extrapolated that into a valuation of Mr Zuckerberg's personal wealth. (Then again, the need to gauge and rank people's personal wealth is a bit beyond me TBH.)