Facebook is the worst of the lot. The UX of groups for niche interests is terrible: hard to find/follow conversations, some messages get randomly hidden or highlighted, you can never go back to where you were in a conversation.
Yet, for some reason, years ago most niche communities I belonged to ditched perfectly serviceable phpbb forums and moved to Facebook. Now there's a second exodus to Discord, as braindead as the one before.
And I truly don't understand Reddit to Discord. It has little or nothing to do with moderation or disagreement with Reddit policies, mind you. At least the ones I've seen, the reason is as vague as "wouldn't it be cool to have a Discord?". Why is never answered clearly.