The social psychology of Facebook?

John Kirriemuir wrote a casual entry in his blog about the “psychology of Facebook”. It is a lighthearted piece, but thoughtful. He suggests various informal hypotheses about why they spoke is succeeding, focusing in particular on the effort people make to grow their networks.
I would like to start learning about social psychology theory and what it might usefully say for incentive-centered design of information systems. My expertise in ICD is largely grounded in individual utility maximization and game theory. I have been saying for the last couple of years that “social motivations” are clearly important for some of the fundamental issues (motivating people to contribute to public resources, motivating them to make effort sufficient to generate high-quality contributions, and motivating them not to misuse and open access platform for unintended purposes). But other than my instincts and casual observation, I have little to go on.
Kirriemuir is not a social scientist (and is clear that he is not claiming to be), and his article is also casual. The social motivations he suggests are not clearly enough to find to test them or generalize to other settings, and his analysis is ex post description, which really does not serve as explanation (in the sense of enabling us to predict or successfully designed in other settings). But he asks good questions, and I think he is right that humans respond to various predictable social motivations in ways that are important for the success or failure of different social information systems.