Traditional usenet clients were the most efficient discussion software in terms of usability. Everything since then (web forums, Slack-like clients, Discord, HN, etc.) has been a step backwards. That’s very much substance in my book.
Perhaps you’re thinking of efficiency in terms of screen/white space or CLI workflows over GUI workflows.
User counts are a poor comparison when you consider how much the pool of potential users has grown.
As points to reference, Microsoft Windows 95 sold 7 million copies in its first 5 weeks so call it ~73 million copies sold the first year.[0]
20 years later in 2015, Samsung, Apple, and Huawei combined sold ~73 million phones across 5 models.[1] Windows 10, released that same year, had Microsoft shooting for it installed on 1 billion devices within 3 years.[2]
2005 estimates 1 billion (or 16% of the global population) online. 2020 estimates 4.9 billion (or 63% of the global population) online.[3] A million users used to be a big deal not that long ago.[4]
Not that I disagree we've gotten better at some things but Outlook Express (newsgroups) and mIRC (irc) seemed plenty accessible to millions of non-technical users.
[0]: https://news.microsoft.com/announcement/launch-of-windows-95... - when searching for a number I saw first-year estimates at ~40 million
[1]: https://en.wikipedia.org/wiki/List_of_best-selling_mobile_ph...
[2]: They made it in 5, https://www.theverge.com/2020/3/16/21116762/microsoft-window...
[3]: https://www.itu.int/itu-d/reports/statistics/2021/11/15/inte...
[4]: https://en.wikipedia.org/wiki/The_Million_Dollar_Homepage ; https://www.youtube.com/watch?v=4e0n7vTLz1U
This was my experience as well decades ago. Unfortunately, people claim that Usenet and/or irc are to technical for the average user to figure out. I guess the skill level of the average user has gone down in the interim.
Remember the eternal september.
One issue is with mobile UI, where there's really no good way to make per-posting read/unread status practical with just touch (and/or voice) input. But I'd also argue that asynchronous discussions (as opposed to chats/IM) are best performed on desktop. On mobile they will always remain a compromise in usability and content quality.
One troll in a community of 200 has an outsized impact.