I am not a very social person. I only use it to follow people from discuss.online. I am fine with you closing it down, if it isn't getting enough usage.
If you do replace it with something lighter, I'll probably sign-up, follow the same people (or more, if they sign up), and keep lurking (with occasional reply).
As for domain, I am in favour of subdomain of discuss.online, but if you want it to be a general user instance, then I think people prefer shorter domain names, not sure.