Now I'm confused. I had never seen Diaspora before in action. It's exactly like Google Plus. But it is from one year yearlier!
Google started Google Plus about 3 years ago.
And from what some Google staffers posted, the basic UI and functionality has little changed from the earliest versions.
I think the UI is just something which time was ripe. And basically, both G+ and Diaspora are "Facebook done right" UI's.