Measuring the Unmeasurable: Estimating True Population of Hidden Online Communities

The true size of hidden populations is an important aspect when staging interventions or devising policies, yet is inherently difficult to obtain due to its nature. In this paper we present a novel approach for hidden population estimation by leveraging activity measured on underground forums. The proposed method consists of two main components. First, we determine the overlap of populations across forums by evaluating users' behavioural patterns. Subsequently, we employ a Bayesian model tailored for extrapolating data from multiple systems in order to estimate the actual population size. We estimate the true number of people participating in online discussion to be 2-8.5 times higher than observed on major cybercriminal forums, and 1.5-3.5 times higher than observed on extremist forums. Our research contributes to a deeper understanding of fringe populations and offers insights into the potential magnitude of participation in online forums beyond what is readily apparent.