Furthermore, aligning on a definition is only half the battle. If you create some arbitrary line (100+ = public), forums will break up in into many groups of 99

Furthermore, aligning on a definition is only half the battle. If you create some arbitrary line (100+ = public), forums will break up in into many groups of 99. Or it will switch to viral messages that are forwarded in 1:1 threads. There is a the fundamental asymmetry between bad actors (who are highly sophisticated) and regular users (who are generally unsophisticated) that makes it really hard to roll out tools / rules that have both high precision and high recall.