[ijk]

Google Groups didn't help, since the interface went from decent to degraded to useless to now-I-can't-find-it over the past decade.

There are still active newsgroups with regular postings, it's just nearly impossible to find them from the web. (And the spam is still a problem.)

[tptacek]

By the time Google bought Deja, Usenet was already dead.

[ijk]

I disagree--Usenet was mostly dead. As I said, though it's not like the old days, there are still newsgroups that I follow, and topics that still have active discussion via Usenet. The bulk of my Usenet reading took place post-Google, so active discussions continued long past that point.

Actually, checking it just now, Google's updated the interface yet again, and this time seems to be an improvement, at least on the spam-fighting front.

[unimpressive]

What newsgroups are still alive? Just knowing where to start would be a huge help.

[ijk]

I can't really speak to general interest. As I said, discoverability is terrible, so I only know of stuff that either I've been following since the '90s or I've seen mentioned by others. So this is a very eclectic list. But they all have recent discussions going on.

  rec.arts.sf.written
  rec.arts.int-fiction
  rec.games.int-fiction
  rec.games.roguelike.development
  rec.games.roguelike.nethack

And a few more active ones I found while searching:

  rec.games.pinball
  rec.games.bridge
  rec.arts.books.tolkien
  rec.arts.comics.strips
  rec.arts.drwho
  rec.arts.drwho.moderated

If you know the usenet hierarchy, you can enter the start of a set of groups (e.g. "rec.games") and the current Google Groups search interface will pop up a list of Usenet subgroups after a sec...but only if you're not already within a group. But the results look to be roughly sorted by recent activity, so that's at least helpful in revealing which of the groups are pretty much dead. (Undead spam threads are pretty common too, unfortunately.)

Note that Usenet quality has always varied (it's the place that more-or-less introduced trolls, viruses, spam, and flame wars). I really can't tell if the quality of the posting has gone up or down. I definitely can't guarantee what you'll find.