Yeah, I don't think there's some obvious number we can use to quantify the success of the Fediverse. It's more of a feeling. How often do threads feel like they have good discussion? How many niche communities are available to you?

Past a certain point, more comments in a single thread doesn't do much. You'd almost never read all the comments in a front page r/AskReddit post, for example. That's too many comments on the same topic and past a certain volume, quality comments can't rise to the top anymore, anyway. But there's so many niche communities that don't have enough people here yet to take off. Especially local ones.