Researchers plunder social networks, by Jon Stewart

"We're harvesting information from Facebook. We have all the information on an entire class of students. We are gathering that data and transforming it into a dataset that can be easily used for all kinds of analysis" said Andreas Wimmer, professor of Sociology at UCLA.

The social sites help map who knows whom and what they do"It's a wealth of data on who relates to whom, and who becomes friends with whom, that is quite unprecedented. Compared to the usual survey data it's a huge leap forward in terms of the precision with which relationships are recorded," he said.
Facebook users can set privacy controls, to limit who can see their profiles. But researchers say that currently most of the students they are looking at haven't chosen to make their sites private, and therefore they are knowingly making details of their lives public.

"There's just a phenomenal amount of work being done that takes advantage of what I would call this passive, massive, data collection effort" said Professor Nicholas Christakis from Harvard University.
http://news.bbc.co.uk/2/hi/technology/7187680.stm