Basically a Directed Acyclic Graph on top of the DHT should suffice. Every node in the graph is signed and links back to two things: the node it is referencing (eg it is a comment to a post) and the last contribution of this user.
Each contributor is announcing their last contribution (and probably reannouncing the last contributions of their "friends"). So finding new content is just a matter of listening to friends and following bread crumbs from there.
