Alot of good thoughts there- I especially like the idea of a "city" with connected but culturally distinct social areas.
It's something I actually like about twitter- you would not think that it's "boost and trend model would be good for this- however the fun of exploring and discovering interconnected subcultures there is really high.
I love making "lists" of different groups and then "visiting" to see what is going on there. Other models seem to lake the organic essence of that.