I happened on a Proceedings of the National Academy of Science story on geography and population density in online social networks.
The article on the PNAS Web site states, "In online communities and other social networks, [scientists] report that any two members can usually find a connection with only a few degrees of separation, partly because individuals tend to optimize friendships geographically. Sociological experiments have shown that social networks are often ‘navigable small worlds,’ in which any person, given only meager information about an arbitrary target person’s geographic location and occupation, can likely transmit a message to the target through a short chain of intermediate friends."
The researchers used data from 500,000 members of the LiveJournal online community, who made available their state and city of residence, as well as a list of other LiveJournal friends. Check it out!