[adolgert]

This multiplicative kind of model is happily amenable to time series analysis, so you can do stats to see what your numbers are and how well they fit. That's great. What's less great is the model quality, given that well-tested virality models can be found in other venues. Coffman looks at this, for instance, at http://datacommunitydc.org/blog/2013/01/better-science-of-vi.... The difference in these two types of models is that symmetries in the statement of the problem permit, or exclude, classes of solutions. Those symmetries come from assumptions about the contact graph, the most basic (and testable) assumption.