The closest thing I've found to USENET, unsurprinsgly, has been Reddit. But it's still a poor facsimile. I spent hours a day, every day, on USENET in the 90s. I'd scroll around the feed of group names and see things like alt.sex.bald.captains or alt.wesley.crusher.die.die.die and have to go check it out.

I haven't spent much time on Reddit, but for me, Hacker News is close to USENET in terms of signal/noise. It doesn't have the subgroups covering every topic, but for what it does cover, it's a great source of interesting information with minimal noise. The karma system seems to help a lot in that respect.

The reason I find Reddit more similar is there are near infinite subreddits and they vary wildly in quality. That was exactly my experience with newsgroups