I'm not sure how we should expect organic comments to scale with posts, and I suspect some of your disappointment is from an assumption that it ought to be linear. I don't have data, but my impression of every other platform I've been on is that that isn't true: comments per post have been much higher in early adoption than in platform maturity. It's too bad we're the first people to be doing this completely out in the open, and don't have anyone else's data to look at.