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I think you've hit on a good point - you want enough users for there to be engagement, but you also don't want so many users that people feel like they're a face in a sea of people that gets crowded out. What the appropriate number of people is depends on the culture and aim of the site more than being a static number to reach. Beehaw, for example, is trying to focus on creating a sense of community and connection rather than growth at all costs. That means Beehaw's "critical mass" is going to be lower than something like Reddit, where it's more of a free-for-all that seems to be trying to appeal by being a "loud" public square type space. Meanwhile, the tiny forum in the corner of the internet about a niche subject is going to have an even smaller goal because it's safe for users to assume that there won't be as many people and, if they're seeking out a forum for a niche subject, it's also safe to assume there won't be as many lurkers.