2. Publicly traded companies have instant liquidity on many more shares, which makes using "last share sold" an meaningful metric.
3. When only 3% of the money a company is supposedly worth has been moved around, it's a poor indicator of what the other 97% would go for.
4. They haven't figured out how to make much profit yet. And it's still questionable whether they will. MySpace supposedly also just needed to turn on the faucet, but apparently the water ran out before they got to it.
5. Oh, and New York smells. (take that!)