1. The Hotmail story, along with many other examples, is recounted in Adam L. Penenberg's Viral Loop. For more on Hotmail, also see http://www.fastcompany.com/magazine/27/neteffects.html
2. For more on the four customer currencies of time, money, skill, and passion, see http://www.startuplessonslearned.com/2009/12/business-ecology-and-four-customer.html
4. This is the lesson of Geoffrey Moore's bestselling book Crossing the Chasm (New York: Harper Paperbacks, 2002).