Alternatively, you could limit your population of interest to the largest group of people that can all find a route to each other. In practice, this just boils

Alternatively, you could limit your population of interest to the largest group of people that can all find a route to each other. In practice, this just boils down to excluding "uncontacted/unknown" peoples. It's a bit different than choosing a percentile to target, and also a bit less arbitrary, in my opinion. If they're going to be referencing the six degrees of separation theory, what they should really be doing is making the same assumptions that were used in that theory (whatever those were).