I'm throughly perplexed about the machinations that allow one social network to dominate another. The question of how to obtain and maintain a user base of millions of customers is one I don't think a good answer exists for.
LinkedIn is one of the few I think actually has a chance at answering the question. It answers the initial questions of delineating access level and social graph noise by focusing on professional relationships. You don't wonder if your boss is looking at your LinkedIn profile, you want them to. You can request recommendations, list qualifications and professional associations, and network with potential partners.
This strong sense of purpose I think will make LinkedIn a contender for survival in the social networking sphere. You can catch my profile there, drop me a line, or check out my professional history.