Bowling Alone: America's Declining Social Capital. Journal of Democracy, July 1995 (abridged version).
Working from the premise that "social connections and civic engagement pervasively influence our public life," Robert Putnam surveys trends in social capital in contemporary America.
The Pew Internet & American Life Project released a report in 2006 on "The Strength of Internet Ties" -- an survey-based analysis of how people use email and the internet to maintain social networks and generation pathways for dealing with life's big decisions. Read the brief press release, and skim the summary of findings in the full report (pp. i-ix; it's a pdf file).