Mark Graham sent me this article by Esther Dyson on social networks and their uses.
As part of a long, insightful essay, Dyson writes:
"There's a real danger that the whole field and its potential for supporting human connections could be irretrievably tarnished by privacy issues -- either as a result of policies that leave people feeling exposed by the aggregation of data, or by security breakdowns, resulting in some kind of informational oil spill.

For now, no one online social network has enough heft to matter. But these issues will inevitably arise when the services approach critical mass. Consider the undercurrents of discomfort already swirling around Google because it is perceived to control the content we see. Imagine a service that controls information about people, even if it only runs algorithms. "

This is a terrific essay with many good points--thanks, Mark!