I don't see why. While everyone you want to talk to is on Facebook you talk Facebook. When a group of friends you want to privately share with is largely on Diaspora, you use Diaspora. Diaspora's focus on making setting privacy levels unthinkingly sounds like it'd be a real useful thing to have hooked into Facebook [for people who give a shit about privacy, but not enough to fight Facebook settings afresh each post].

Embrace, extend and ...