Usenet has/had hundreds of millions of active users?!?

That's certainly news to me.

I can’t find anyone who says how many users, but 110-120 thousand newsgroups.

With a tenth active users and ten times the spammers!

Which is just absolutely full of spam now, plus you need to pay for access (largely).

How about "email"?

Do people forget it's a common protocol that isn't monopolized by anyone and is the widest prevalent even today, perhaps because of it?

I don't think many people consider "email" to be a community in any meaningful way. The same way people don't consider physical mail, the telephone network or the internet as a whole a community.