In February 2003 - some time after the bubble burst - Amazon.com had a market capitalisation (US$8.4bn) equivalent to 40% of General Motors. Yahoo! had a market cap of over 50% of GM, while eBay had a market cap of 110% of that of GM. That's been attributed to behavioural finance - essentially investor perceptions that they'll be able to buy in time to enjoy significant capital appreciation and sell before their peers drive the market down, in other words a manifestation of herd behaviour.
For a technical discussion of behavioural finance see in particular Andrei Schleifer's Inefficient Markets: An Introduction to Behavioural Finance (Oxford: Oxford Uni Press 2000) and Bertrand Roehner's Patterns of Speculation (Cambridge: Cambridge Uni Press 2002).
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