The 'old' Internet is shrinking and being replaced by walled gardens over which Google's crawlers can't climb. Sure, Google can crawl Facebook's 'public pages,' but those represent a tiny fraction of the 'pages' on Faceboo, and are not informed by the crucial signals of identity and relationship which give those pages meaning.
I like Diaspora because it's audacious, it's driven by passion, and it's very, very hard to do. After all, who in their right mind would set as a goal taking on Facebook? That's sort of like deciding to build a better search engine - very expensive, with a high likelihood of failure.