I'm not going to suggest "subreddits" as a solution because eliminating mods in favor of ignoring/following makes moderation more decentralized and likely that Hubski will avoid the authoritarianism of reddit. In place of subreddits Hubski has "tags" and "following". This makes Hubski user-centric and thus a friendlier place than reddit. Reddit is largely the democratization of thought, not discussion for its own inherent value.
Back to my main point; sometimes I feel I can't share something because it may not interest all my followers. My followers interests all overlap with mine in different ways. This is because friendship is not a binary. People talk about different things in different contexts and with different groups of people.
Tags are also not a solution because anyone can spam a tag's stream and then we're back to subreddits with authoritarian mods. So interest/discussion-context clusters do not match users or tags.
So, how we need an algorithm that recommends posts to users through their overlap.
Where users' posts overlap there is formed a discrete interest. The transitive property can be used to connect these overlaps as they are actually the same interests.