The Pew Internet and the American Life Project is a great source for data on Internet use and trends--sometimes specific to gov, but mostly about everyone. Their most recent report is on "Adults and Social Network Websites." The data may blow away some of your assumptions.
Social network users are equally likely to be men and women and more likely to be young.
- 75% of the online adults between ages 18 and 24 use social networks
- 57% between 25-34
- 30% between 35-44 year olds
- 19% among 45-54 year olds
- 10% among 55 to 64 year olds
- 7% of online adults 65 and older.
- 48% of online Hispanics adults have a social networking profile
- 43% of African-American adults
- 31% of white adults