The official chatrooms here are always an option. The interface has a lot to be desired, and it isn't very active, but you can often find someone there to chat with, especially if it's about Stack Exchange.
The site's blog has a Discord, but there's even less activity there than in chat. (For example, there were a total of twelve messages posted there in the past six months.)
There's a Slack group started by the late KutuluMike that has a number of this site's long-time users (ten of the top thirty rep users are on it).
It was created a few years ago to give a forum for more open discussion than Stack Exchange chat allows, and has been steadily continuing since, with a good 5-10 users active there on any given day, and I've found it a to be a great community.
Pretty much all topics of scifi are welcome to be discussed, both related to this website and not.
You can also just head to a subreddit like /r/scifi. You won't find much overlap in the userbase, but you'll certainly find a lot of activity.