The US Pew Internet Project has published a report on social media usage by young adults – read here the overview:
“Two Pew Internet Project surveys of teens and adults reveal a decline in blogging among teens and young adults and a modest rise among adults 30 and older. Even as blogging declines among those under 30, wireless connectivity continues to rise in this age group, as does social network use. Teens ages 12-17 do not use Twitter in large numbers, though high school-aged girls show the greatest enthusiasm for the application.”
You can download the whole report as a pdf-file.