This is interesting.

Obviously, I don't have stats on things like the % active accounts vs inactive and such, so this is pure speculation.

If you look at the hot sorted posts on r/GothStyle they seem to get around 100 or so points per post. Note, this isn't a direct translation into upvotes. It also says there is 145 people online - Does that mean roughly 2/3 of active users vote stuff to hot? ~100 people holding up a niche community with a fraction of those the posters themselves.

So in effect ~0.1% of a subreddit's subscribers makes things happen. I always baselessly suspected that Reddit fluffs up the numbers to make engagement seem like it is much greater than it is, but this is 1000x smaller than the sub count suggests.

I'm sceptical of my maths here but r/PCMasterRace is similar. Out of nearly 8mil subscribers, roughly 8000 online.

Doesn't really mean anything. Facebook has around 3.4 billion active monthly users. Reddit has around 400 million. I'd still take the latter than the former. Lemmy will keep growing. Probably will never have 100s of millions of users and that's fine. More users can be a good thing but by itlself the numbers mean nothing.

What do you mean? They can access the contents of each other

Is there any benefit to merging if it all just federated between them?

probably not, since kbin isn't Lemmy.