I love this post - and I personally blog quite a bit about the importance of giving community members tools to define their own subspaces. I find that one super large community can create worry in leadership when engagement ratios start to look too thin. Even if overall engagement is increasing, if member growth outpaces it, your top-line conversion # can look worrisome. I much prefer offering a platform for people to create their own communities inside of it where they can feel more ownership.