[zulip] <Brenhin Keller> Oh for sure -- the Zulip is also in that category (open source, allows self-hosting, free hosted instances for OSS projects), and has nice support for LaTeX, which is important for the mathematically inclined. But a lot of people in the Julia community either like Slack for some reason, or (like me) are on it a lot because our workplaces use Slack, so that's the single largest forum. Zulip is probably the second larges.
[zulip] <Brenhin Keller> Oh, and the discourse is large too -- but that's not chat
[zulip] <Andrés Riedemann> Also, not everybody is subscribed to general-bridged