Gregory's ServerNot sure exactly what Eugen is using, but the user counts are easily accessible via the API and it wouldn't be too hard to run a calculation for all the instances which federate with mastodon.social, which is most of them. Won't be (totally) complete, but I'm not sure Eugen is claiming that.
@carl @Gargron great answer, seems likely. was just wondering whether there was another channel / phone homing behaviour built in , which would be potentially problematic for privacy but useful for rolling our emergency updates etc