Throwing eTrade as an example is horrible since guess what? eTrade has 0% affect on the price of ANY stock. eTrade can shut their site down at any moment but the markets will not give a shit.
QFT. The price of a stock is backed up by the underlying assets and the future earning power of the company it represents. E-Trade isn't even an exchange, it's just a broker and there are 58 billion other brokers eager to take its place if it were to ever implode.