I've see this called the 'Warrens' model. Reddit does this with Sub-reddits. Keep the niche small group feel available, while aggregating useful stuff upwards.
Unfortunately I'm unable to find the reference. It's probably a different word, but the idea is the similar, and I specifically recall it referencing Reddit and other communities with a similar "small group" branching model. Warrens came to mind because of rabbit warrens, but that's probably not quite it.
