Zulip really hits the sweet spot for a communication tool. Especially with large amounts of users communicating across many different threads. The FHIR community "chatroom"[0] is a Zulip server and is indispensable for communication and knowledge building across thousands of users.
I signed up for that to see what Zulip is like, but there are only 19 messages visible if you do that, so I shouldn't have bothered. It seems nice, I guess.
[0] https://chat.fhir.org/#