Reddit successfully killed most if not all web forums, and reigns supreme over that. Nothing even comes close; but some twitter/discord groups might be the beginning of the end.
I don't understand either of those. With twitter there's no room for thought or nuance. It's just a basic "hot take" machine and that's it. Sure, Reddit is full of those, but there's some depth there too at times. I understand discord even less. When I look at it it seems like just a giant chat log. I can't easily find new insights from overnight without rewinding the whole log and parsing it all. I don't know how to pick and choose topics that interest me, it's a fire-hose of noise. I assume one can do it, but I can't figure it out.
I disagree about your view on Twitter. I'd even go as far as saying it's better than Reddit for nuance because going against the pack doesn't cause you to be downvoted into obscurity. It allows more than one view. It's great for interacting directly with the researcher that did an interesting study or the developer who released a new software.
The common issue for all of the services are when they become mainstream. It attracts bad apples. Twitter used to be and still have some great interactions just like small subreddits do, but when a post reaches /r/all or the feed of a controversial celebrity, you know it's going to be awful.
Discord is more like IRC used to be. It thrives in small communities of 30-50 people where you talk regularly and each topic has its own channel so regular chitchat doesn't drown out the interesting bits. It's more of a daily pub talk with your friends than finding a tutorial 10 years after it was written.