I'd like to just point out that you seem to have a very good grasp of how this site can function. It's encouraging. I think right now what we're experiencing is just a very small community, once the site scales Blob_Castle, and others, would likely not notice an absence of conversation. "Pushers", like certain sections of a newspaper... I really like the analogy.
I... I really like Hubski.
I have a close friend who loves tumblr and we may or may not spend hours talking about the ins and outs of social networks, the benefits of encouraging/discouraging dialogue, how to defend against Eternal September, and whether or not "Well-Kept Gardens Die by Pacifism" is still relevant. Having some trouble getting my head around Yahoo! Pipes or you can guarantee I would have some cool data for this place.