@misty I've only encountered one public Internet instance (and can't say that I've "used" it) – our local FirstClass distributor still runs a public server to this day (firstclass.lt), though it's practically empty aside from generic FC news.
(We did have an inter-FC email exchange set up between it and our internal instance...)
@misty Yeah, I think they were more about (re-)selling it as an intranet product than building any sort of public community. (When I peeked at its user directory, it looked more like one admin's "personal/family" server than anything else.) I believe it still has open registration though.
We only barely used our intranet instance (and the licenses cost a lot for the small local uni/college); by 2014 it was still up and I was setting up Active Directory sync, by 2016 it was powered off, everyone had already been switching to Moodle anyway. (Now I run an empty "retro lab" instance on the same license.)
I know that a large university was an extensive FC user though – at least I got that impression from outside; their primary student login system was still backed by FC until ~2020.