“HIV/AIDS, like many other contagious diseases, exemplifies the common view of so-called viral propagation, growing from a few initial cases to millions through close person-to-person interactions. (Ironically, not all viruses in fact exhibit “viral” transmission patterns. For example, Hepatitis A often spreads through contaminated drinking water.) By analogy to such biological epidemics, the diffusion of products and ideas is conventionally assumed to occur “virally” as well, as evidenced by prevailing theoretical frameworks (e.g., the cascade and threshold models) and an obsession in the marketing world for all things social. . . .
Despite hundreds of papers written about diffusion, there is surprisingly little work addressing this fundamental empirical question.
In a recent study, Duncan Watts, Dan Goldstein, and I [Goel] examined the adoption patterns of several different types of products diffusing over various online platforms — including Twitter, Facebook, and the Yahoo! IM network — comprising millions of individual adopters. . . . In all six domains the dominant diffusion event, accounting for between 70% to 95% of cascades, is the trivial one: an individual adopts the product in question and doesn’t convert any of their contacts. . . . only 1%-4% of diffusion trees extend beyond one degree. . . . cross the six domains only 1%-6% of adoptions take place more than one degree from a seed node, meaning that the vast majority of adoptions occur either without peer-to-peer influence or within one step of such an independent adopter. . . .
In all the examples we study, diffusion seems remarkably un-viral, rarely spreading far from an independent adopter. Our results thus call into question the dominant, epidemic-like models of diffusion, and also the value of viral marketing campaigns. On a positive note, this observation makes life a lot easier. Instead of needing to describe, predict, or trigger a complicated viral process, one can focus on the much easier case of adoptions that spread at most one hop before terminating. It turns out that diffusion is not nearly as messy as you might think.”
Hat tip: andrewgelman.com