This great survey from the American Marketing Association reveals marketing association members usage of social media. Facebook and LinkedIn are at the top of the list. However, the survey is from 2009, so it would be interesting to see an update.
"LinkedIn is the social network platform of choice for marketing professionals. More than half
(58%) of survey respondents indicate they use LinkedIn, a social network platform targeting
business professionals. Nearly a quarter (23.6%) of respondents prefers Facebook while 11% of
respondents do not use any social networks. A relatively small percentage of respondents use the
micro-blogging platform Twitter (3.4%) and MySpace (2%)."