I remember the uproar a few years back as students complained at the notion of allowing adults on Facebook. But, it seems now that the discussion was SO 2007.

A new study reveals that 59% of all U.S. Internet users, ages 16 - 54, who access the Internet every day or every other day, have a social networking profile somewhere. This is up from 43% in 2008, 40% in 2007, and 39% in 2006. The 16% increase in 2009 from 2008 reflects a trend of significant numbers of adults jumping onto the social netowrking bandwagon.

Source: eMarketer
http://www.emarketer.com/Article.aspx?R=1007232