How Strong Are Network Effects Online, REALLY? (EBAY, GOOG, AMZN)

Network effects. Perhaps no other phrase can get a VC's pulse higher. They're the holy grail of online business. What is a network effect? It's what happens when the value of a product to one user depends on how many other users there are, as economists Carl Shapiro and Hal Varian put it.  Examples include Microsoft Windows and the phone network. Windows is valuable because most other software is...