On facebook, im in 3 different Akita groups. I think the total membership on all 3 is over 70K... the biggest has 33K members and more activity than steemit.

That exists because Facebook has a billion or so members. It didn't get that way by starting as an Akita site, nor did it have Akita groups when it had a few thousand members (or possibly not even at a few million).

Small markets are easier to penetrate, and their users will be potentially far more engaged.

I can't really agree because of the difficulty of establishing any the minimum critical mass for any discussion or community to have meaningful utility. Try to start an Akita community on Steemit and see how far it goes (it won't). Even if you get a relatively large share of the Akita discussion universe, it won't be enough people here to thrive. It may (and certainly would be) difficult to get a large portion of the porn market, but it will be far easier to get enough people on here to even have discussion and interaction occur (in fact there is already enough people).

It is possible once the subcommunities features are better developed, some small subcommunities may organically grow up here (though I still have doubts due to the pressures of the reward algorithm), the way they do in subreddits, but for now I see no way that can work outside of subject matter with at least relatively wide accessibility and demand.