Interesting. The SSC home page itself points to the site's subreddit as the intended place of discussion, but this could work as a "lifeboat" site in case that runs into problems with the new reddit content policy.
Some people just plain don't like Reddit. I'm not quite clear on why, but it predates the current cancellation fights. Perhaps the upvote/downvote system encourages attention-seeking behavior.
I'm a reddit user, and I find that it doesn't encourage ongoing discussion, even in the smaller subreddits - a thread is pretty much frozen in stone after a day or two, even if technically people can post there for six months, so people keep starting new threads on the same topic (often covering a large part of a subreddit's front page with discussion of the same subtopic, preventing other discussion) but without history or interlinking. The problem is that posting to a thread doesn't "bump" it, so if you want people to see what you said the only option is a new thread.
As much as I like Reddit, it isn't a great place for having deep and reasonable conversations about anything nuanced. I largely attributed it to the upvote/downvote system and the pseudonymity, but it's probably as much to do with the moderation and the culture of the site. I do think it's a matter of identity and culture - people aren't there to learn to challenge their beliefs and learn new perspectives and discuss things. It's too easy to go from a post about some white trash pulling a gun on protestors where people are just blindly piling on the issue to a post about whether I should dump my boyfriend because he forgot to pick me up to a post about cute puppies. Compare that to something like SSC or LessWrong (even if it isn't perfect), where everybody knows why they're there and there are significant efforts both from the community and the moderators to uphold that.
I'm one of those people. I'm told the problem is mainly that I can't find the right subreddits to follow; any remotely popular subreddit is a) very easy to find, and b) almost completely worthless. It's really the same problem that this entire HN thread is about!
I've found that single-player games often have shockingly nice communities on Reddit.
r/EnterTheGungeon and r/AceCombat were two great ones. I say "were" only because they (naturally) become sort of semi-dormant because those games haven't seen major releases in a while, so discussion has petered out. r/Xcom is pretty strong!
It's hit or miss, but a lot of "niche" hobbies have really good subreddits. I recently got into watches and subreddits like r/Casio and r/Seiko are fun and supportive. Not exactly deep discussion, because there's only so much you can say about a watch, but they are fun.
There are some decent audio-related subreddits. I'm a mod at r/budgetaudiophile and we try to be as helpful as possible who are dipping their toes into the hobby, though I'm not too active over there these days.
I haven't found any solid tech-related subreddits, ever. Though, I haven't looked too hard I suppose.