Hubski is low-traffic enough that it's easier for me to just skim through global than to pick either users or tags to follow. I guess that makes it Plaza-like for me in his analogy, but just because of the low volume. For small spaces, Dijkstra is as good a pathfinding algorithm as any.
Yes, between that the community page and the chatter page you can make a very strong case for it being Plaza-like. I could also see it remaining so with a larger volume, but I'm sure somebody could come up with a mathematical model to show at what point you switch between the two.
I think in the best case scenario, Hubski behaves as a warren-like space. Chatter still only shows you what people whom you are following are saying (or what people are saying about content that interests you). The global pages invite you to enter into the plaza if you choose. Eventually, I would like to see them done away with, but I think in order for that to be sustainable, the traffic here would have to be an order of magnitude bigger than it is.