Dare Obasanjo has posted some intriguing analysis of what an "open social network" is. Read it here.
Obasanjo is in charge of the social networking aspects of Windows Live.
Here is a pretty complete (if admittedly biased) presentation of what's wrong with closed social networks (e.g., Orkut, MySpace). This is by Richard McManus, who has a business connection to one of the aggregators.
And there's this by Scott Gilbertson of Wired.