Using an SIR model seems a shady approach for this model. The model inherently assumes an amount of time where a person is "infectious." They modified the traditional SIR model, Also, they never allow migration from the Recovered class to the Susceptible class again, meaning that once a person stops using Facebook, they will never use it again. It seems to not take into account age buckets (where Facebook may be more "infectious" with different age groups).