"People seem utterly exhausted by the plethora of networks they manage and the number of people within those networks".
I agree with David and believe social / information / conversation overload is taking its toll on the value of social applications to many users.
While the answer may be the introduction of more advanced "filters" (a la Shirky's suggestion), not everyone will be ready to manage their social streams through finer and finer filters. The "Geeks" maybe… the "Normals" … no way.
To help extract and maintain the valuable "meaning" found within our social networks, we need to rethink how we manage and connect with our most important social hubs… not just filter the raging flow of social streams.