Check this post Social Media 3Q Update: Who Uses Facebook, Twitter, LinkedIn, & MySpace? from Social Media Today:
The third quarter 2010 demographics of Social Media users according to Google’s Adplanner services has a few surprises. Facebook actually dropped from 550 to 540 million users in the third quarter, and Facebook users over age 54 dropped from 16% to 10% in the last six months. Based on the data from the 2nd and 3rd quarters there is a significant slowing in the growth of the major Social Media tools.