In my experience, once a community reaches certain size, it grows organically. But reaching that cricial mass is hard.

So I thought that a coordinated "consented white hat brigading" might help. What I mean is for a group of users to focus on one or a few communities for a while trying to get them to that point posting and commenting (quality content).

It should be done asking the moderators for permission first. There may be communities that don't want to grow this way.

Disclaimer (because I got a comment in a similar post saying "Give it time"): I'm not trying to rush the growth, IMHO people should post as much as they want and not take it as a chore. I'm thiking of focusing the (natural) activity of those users interested in helping small communities (that want to grow).