It depends. The Equity should be based on future expectations of return. For Amazon and Google, there are now clear business models to drive their value forward (for Amazon it may be rather longer term than for others) but for startups it is less obvious. Most of them have no idea how to generate value and how to grow forward, and their actual "utility" as a service may be questioned.
The trend towards over-valuing the startups currently is also coming from the fact that the housing bubble has exploded and investors and putting their cash in other fields where they expect to earn more/lose less money.