Jason's third point is, I feel, the strongest. People aren't contributing to Diaspora because of Diaspora, they're contributing as an act against Facebook. The enemy of my enemy is my friend and all that.
I'm not sure if that's a good thing. It's one thing to get in front of a movement and ride that to success. It's another to get in front of a movement that's predicated largely on hate. Diaspora would have to turn this into something positive (Internet users for privacy!) as opposed to just letting the anti-FB vibe run its course.
PS: not related, but here's a thought: why is it necessary to have a special node system? Won't a super-simple, super-private social network work just as well? Facebook's problems isn't so much with the tech as it is with the company philosophy - they seem to want to do everything in their power to screw you over for their benefit. Change the philosophy and Facebook becomes a much harder target to hit.