I've found (thanks a a twitter post by Michele Costabile) a very nice article by Marshall Kirkpatrick covering the effects of recent troubles of some social networks (like Twitter), acquisitions of others (FriendFeed), and the overall evolution of the social networks. The post is at www.readwriteweb.com/archives/is_a_perfect_storm_forming_for_distributed_social_networking.php. 

I have to say that with social networks asking for full ownership of the contents you post, disabling accounts they feel illegittimate, and not providing enough confidence in being always online, it would make sense to build the "social" nature of the Web by aggregating separate sites, rather than letting one site own the content. We'll see.