Just looked at the new bitcoinx website, it shows:
Bitcoins sent 24hr: 784,899.31 BTC
MTGOX 30 DAY volume: 22,288.79 BTC
784,899x30/22,289=1056
So the exchange of BTC towards USD take only 0.1% of all the BTC transactions, it means if most of those BTC are going for sell, the price will fall like a knife
Does your transaction volume account for self-sent change in each transaction?