I've mentally categorized the 3 different networks as, respectively
- where I connect to people that I might at some point in the future ask for a job
- where I connect to people who take photos, mistakenly think I care about their taste in music, and constantly update their statuses (stati?), and
- the annoying 3rd one that I wish would go away
So, the problem isn't that there are too many social networks, but rather that there is no standardized mechanism by which 2 users can determine whether they share a SILIIC and could therefore safely drop it from their list.
If everybody were to list their social networks in a ranked metadata format, bots could crawl the links and suggest a path towards simplification, e.g. "You and Bob have both listed LinkedIn as your preferred SNS, you can safely ditch Plaxo."
This model allows users to continue to actively manage their social network in duplicate places (which they clearly enjoy) but keeps this number manageable, i.e. approximately 4-5.