After talking with evan a bit, I think I know where my hmm'ing comes from. Not a fully cohesive thought/suggestion/objection or anything, just a hmm =)

If the major load of LJ is in the Friends View, then the best split of the users into clusters will minimize cross-cluster communication for the Friends View and Communities.

However, there's no good heuristic on how we make our friends. evan's friends are mostly in his geographical area. Mine, however, are not - online-wise, I'm actually less likely to know people in my own area.

LJ needs an architecture that deals well with randomness of its users.