the circle of closeness
The accounts you reached for most over the last month, ringed around an arena. Posts get dealt into the middle; you decide who wrote this. Twenty cards, a grade against blind guessing, and a card you can copy. Then, if you want the expensive answer: analyse the circle and read the whole-history closeness matrix out of their repositories — who spoke to whom first, and how much since.
Mutual follows fill the ring first; if there are not enough, the closest non-mutuals top it up and say so.