Interesting. Hadn't thought about the amount of posting done on hexbear. Do you know why they kinda defederated and are now re-federating?
Otherwise, another factor is the effect of the wider fediverse. Interconnectivity between lemmy and masto is not great, but I'm seeing more attempts at it and interest in it from both sides, which is cool. And people are likely to create accounts from either side too, which for lemmy/kbin means that millions of people are slowly learning about them with an already vested interest in fediverse platforms succeeding.