Then there are Reddit-style discussion groups. The older system is Lemmy (but I don't recommend it https://mstdn.social/@feditips/106835057054633379). The newer system is Kbin (https://kbin.pub, https://kbin.social) which is less problematic, but it's still in its early stages and the interaction from Mastodon still seems a bit flaky.
So, at the moment the Guppe and Chirp groups seem best, but there should soon be more choice.
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