Stocks are priced on the margin, which means the last sale sets the price, and so the market cap for the whole float. The $440B never really existed. But it could have been used as collateral for loans which would turn it into 'real' money. However when the price falls, those loans may be called in, and so the 'real' money is destroyed. If these loans have to be be paid off by selling the stock, then we enter a deleveraging spiral.

Theoretically the market cap should equal the present value of expected future returns (dividends and buybacks, not speculation), modulo risk, but that assumes a perfect market.