https://eternal-september.org/ last I checked there was still some activity on comp.misc after Slashdot pissed everyone off with their Beta a decade or so ago (same time Soylent News spun off as well). Definitely a few others with a handful of posters.
But yes, it's definitely small islands in a sea of spam or just dead groups.
It got ridiculous pretty quickly. The overhead to spam was so low as the protocol was designed to be low friction for posting. The system then took care of carrying the payload everywhere in a reasonable time. People fought back with filters and kill lists. But was not really enough.
Once the ISPs decided they did not want the added cost of running the servers usenet tanked pretty quick. Still alive here and there. Not even close to what it could have been or even was.
Surprised someone has not made a mastadon to usenet transfer protocol. It almost fits both projects goals.
I grew up with BBS access for a number of years, but no USENET access.
When I finally got access to USENET ... what a terrible place it was! SO MUCH SPAM.
And the few newsgroups not riddled with spam just had poor behavior. The nice thing about BBS conferences were they were all moderated. And the ones I was part of required you to use your real name (as verified by the BBS sysop). They took it seriously - if a sysop was found not to be compliant, his BBS was kicked out of the network for a period of time.
The only good thing about USENET was the tooling (news readers, etc). Otherwise, both early web forums and BBS's had it beat.
Little bit of both. From my own anecdata, most people I knew left usenet due to spam problems. Most of the people who did not were primarily the ones using it for binaries. And then yes, the binary angle started the trend where ISPs stopped offering it altogether, which even further reduced the likelihood that people would use it.
And then there were weirdos (sickos?) such as myself who hung on for an absurd amount of time and never once used it for binaries
I had to selectively stop carrying parts of USENET (I ran what was briefly a fairly important node globally) because of the volume of the binaries, and various sex and bestiality groups (probably still including some badly-scanned ASCII-rendered images!
What we have today is drastically, unquestionably better that what Usenet offered. The very fact that we're conversing in real time in a coherent thread where everyone sees the same messages is a basic task Usenet was not fit to provide.
In the early days Usenet propagation was slow and haphazard because the communication links available were very limited. Nowadays I can post a message on one Usenet server and it appears on other servers in a few seconds. So coherent real-time conversations are no problem.
On the other hand, with a long-running discussion, HN, Reddit, etc. still have no way to see what messages are new since you last looked at a thread, something which Usenet clients have always done and still do now.
Usenet is a system so bad that "posting a message on a Usenet server and having it appear on other servers in a few seconds" sounds like an achievement. And: both those other systems have reliable ways to see all the new messages on a thread, unlike Usenet, which couldn't even guarantee that you'd see all the messages, let alone in order.
I was a Usenet systems engineer (regional ISP operator, INN hacker) during the heyday of Usenet, and a dedicated user in that time as well. These rose-tinted views of how well Usenet worked don't fly for me at all. Reddit is actively, materially, multifariously better than Usenet, and Reddit is not the state of the art.
But yes, it's definitely small islands in a sea of spam or just dead groups.