We have used for a while our own instance of Rocket chat, at https://chat.karrot.world/ (at the time of writing, until it redirects to the new chat).
Then we decided to do some dogfooding in parallel and created our own Karrot group. We also did it to make it more accessible to users that are already on Karrot.
Since we created our Karrot group, the chat usage has declined substantially, so now we moved to Matrix, at https://matrix.to/#/#karrot:matrix.org
The old chat will be closed down on 30th November.
Some reflections on the tools we use, including Karrot
Starting with the question Why do we need a chat, after all? Isn’t it enough with this nice community forum? Too many places to be in, too much information to keep track of! 
I think this kind of forum (Discourse) has been really nice not only to engage with the communities using Karrot, but most importantly as a way of keeping information information organized and searchable.
On the other hand, a more dynamic, ephemeral chat is also very useful and needed, for the purposes of onboarding people, talking about development and whatever needs to happen on a quick pace. Leaving Rocket Chat to Matrix comes with a few advantages: many developers are already on Matrix, so it’s easier to keep in touch and less of a barrier with a new signup, plus no need to host it ourselves. On the other hand, it seems like we might loose our github hook and the notifications for when new groups are created on Karrot (maybe not… Nick is looking into that).
What about Karrot? For me it feels much more like home in comparison to the other chats, but I must admit that it doesn’t have that sleek and fast quality of messaging as the other apps do. It still feels a bit heavy somehow, but I don’t feel it compromises the quality of conversations.
Another nice thing about it is to keep track of meetings and who’s attending them. The main thing about it though is engaging with the people from other communities who are also interested in Karrot design and development. It should be interesting to see how it evolves with time, in terms of helping us organize our efforts as a team or bunch of people who are using, conceptualizing, designing and developing Karrot. In other words, assisting the self-organization of a community. That is what Karrot is built for anyways! And this is when we start to get into some interesting meta territory…