This reminds me a lot of the disease models[1] I studied in grad school. Say you have a population consisting of individuals who are either Susceptible, Infect

This reminds me a lot of the disease models[1] I studied in grad school. Say you have a population consisting of individuals who are either Susceptible, Infected, or Removed (dead or immune). The goal is to find the percentage of the population that will be infected over time. The peak of the infection, as well as the length of time it's around in large numbers, depends on the ratio of two key parameters: the rate of infection and the rate of recovery -- or, phrased as the article did, "virality" and "retention" ( =1-recovery ). If you're relying on a viral product for revenue, then, tracking both of these factors is tremendously important. [1] http://en.wikipedia.org/wiki/Compartmental_models_in_epidemiology#The_SIR_model http://en.wikipedia.org/wiki/Compartmental_models_in_epidemi...