I assumed the concept had come from Google's own research, which I first saw a year ago in this slide deck: http://www.slideshare.net/padday/the-real-life-social-networ...

According to Wikipedia, Diaspora was initially funded in April 2010 and a developer preview was released in September 2010. The Google slide deck was published in July 2010, so presumably this is an idea they had been kicking around for a while before then.

It seems that the notion of friend (circles|aspects) is just one of those obvious ideas that everybody had, but nobody executed properly until now.