Some niche communities are lacking, though I don’t see them existing/growing without a large increase in userbase, since they’re you know, niche. Cities for instance… some I used to frequent on reddit exist on Lemmy, but are naturally not as active. Also I used to talk in (hooray) a couple subs about chronic illnesses which are like, 1% or less of the population, maybe more since sometimes it would be family members or parents talking. Celiac, type 1 diabetes. Same as cities there are communities about those but they are not very active yet.
Dedicated users posting is the best way to slowly grow communities like that. It's much less likely others will post if it seems like a ghost town, but if someone is posting others are much more likely to join in. And at the very least there'll be content to engage with when new people find it.
yeah, for example.... when reddit was beginning they made fake accounts and posted to pretend there were users. Thankfully lemmy/kbin/mastodon etc are past that point, for the most part, but for certain communities I might as well be there talking to myself. I don't mean that as a bad thing!