It is important to remember exchange rates are not static. The exchange rate is determined by the supply and demand of the two currencies for exchange. If you suddenly liquidate a lot of BitCoins all at once, you will raise the supply on the BitCoin side and thus drop the BitCoin-to-dollar exchange rate. It is not valid to take the exchange rate, multiply by the number of extant BitCoins, and declare that to be the size of the BitCoin economy in dollars; that will grossly overestimate it.
(This has nothing to do with BitCoin qua BitCoin, it's the nature of currency exchange. Because most currencies that people deal with have such enormous economic bases, people tend to reason about them such that they neglect certain terms that they don't have to worry about in practice because of sheer size. If I exchange $100 to Euros, I won't personally budge the exchange rate noticeably. But in a much smaller economy, you can't neglect such things.)
(This has nothing to do with BitCoin qua BitCoin, it's the nature of currency exchange. Because most currencies that people deal with have such enormous economic bases, people tend to reason about them such that they neglect certain terms that they don't have to worry about in practice because of sheer size. If I exchange $100 to Euros, I won't personally budge the exchange rate noticeably. But in a much smaller economy, you can't neglect such things.)