We've been using Zulip internally for a couple of years now. We've used IRC and Jabber, looked at Slack and Hipchat and Skype and Lync, and somehow keep coming back to Zulip. It lets us have real, ongoing, and substantive conversations, with a large number of participants, without being overwhelmed.

I sometimes feel like Twitter is actually a better comparison for Zulip than Slack---in Zulip like on Twitter, it's easy to watch and participate in multiple conversations at the same time. Zulip's threading model exists somewhere between Slack's rigid "rooms" model and Twitter's everything-is-public model, so it's much lighter-weight to participate in multiple places at once than on Slack but it's also easier than on Twitter to have the right conversation with the right people without bothering others with something irrelevant to them. And Zulip's threading model makes it much easier to have multiple conversations within the same space without stepping on each others' toes or getting distracted.

Our remote folks rely on it particularly heavily. When Zulip got acquired it was our remote employees and their managers who were showing up outside my cube with pitchforks when I breathed a word of turning it off. It gives folks in other offices or working from home a watercooler and a way to virtually tap a group of coworkers lightly on the shoulder when they need help.

Basically we can't live without it, so I'm super-excited to see it finally open sourced. Thanks for making it happen. :-)