At Eclipse, we have forum, mailing list, Mattermost, and Bugzilla. Each project and working group has its favorite tool. Bugs on bugzilla are of course used for issues, but also for features and improvements discussions. Live meetings can also be done for small groups on Skype or Hangout. Mailing lists are still heavily used.
I would say that having meetings in Mattermost and just write a summary on Bugzilla or mailing list help a lot to have a cleaner tracking of decisions and discussions. Plus, Mattermost archive the discussion in dedicated channels, and this is more convenient than IRC to retrieve if needed.
In short, the resume of the live discussion is the equivalent of the approved answer on Stackoverflow: all the noise (comments, other answers, ...) is less visible. And it highlights the good content and the best users.
The issue with technical communities like Docker, Openstack or Eclipse is that you don't play with that on Saturday evening. People contributes because this is their job. However, you can post a blog with your Ubuntu laptop about your hobby: this is not the same market, users, contributors.
You have to take that in account. When I was working at Bonitasoft, I had quite the same issue: BPM is not very fun. The main issue I had to solve at the time is that in mailing list and forums, this is very difficult to find the important post: you have to read everything, and this is very annoying.
So we moved from a classic forum to a Q&A à la Stackoverflow: asynchronous but highlights key content/users. We produced dedicated tutorials for specific questions and use case. It helped us to gather our users on our community website instead of other tech websites. Then, we rewarded the most active users: featuring on the blog, specific high quality swag (poster, mugs, t-shirt, stickers, ...), meeting them IRL when possible (when a consultant goes in their area), free access to support, and more. This way, the "average" contributors could "follow the leaders".
Also, this is very important to organise conventions, events or meetups so people can meet in real life. And if possible sponsor important people of the community. Canonical does that for Ubuntu (UDS in the past, UbuCon, ...) and we do that too at Eclipse, with the EclipseCon, Eclipse Days, and other local events. This help a lot to build relationship, nurture the leaders of the community, and find new contributors.
One last point: we are now in an change of generation: mailing list was the top 20 years ago, and some early open source developers are now project managers or retired. The new generation of developers grew up with live messaging, modern tooling, ...
You really need to think about new ways to contribute and attract people. In a way, each community has its own communication stack and you must adapt it from time to time.