Honestly, I think that best thing to do is just pick one or two communities that you really care about and invest a bit of your time. Make a post or two, have some back-and-forth with a friendly person in the comments. You don’t have to be a moderator to help build a community. Even just taking 30 seconds to make one post would be good enough, and might encourage someone else to make a post.
I was always more of a lurker on Reddit, but I don’t think that’s really gonna be enough if we want this place to at least get to critical mass.