I mod a 21 thousand member sub on Reddit and opened a similar one here. I put several relevant posts to get things started and let people on the sub know we are here with a tutorial on how to sign up and explanation on why it is good to have an alternate.
I now have about 20 members, a few coming each day. However no one has posted or replied at all.
Another community I created has a similar result.
I know there is a threshold of activity that stimulates more activity, but so far its not happening. I've started many active subreddits, including one in the 1% with half a million subscribers. I'm not new to community building. I'm honestly concerned.
I want to encourage everyone to post and comment in the little communities you have joined. You will be contributing to this whole project when you do.