The Pew Research Center's Internet & American Life Project has released Why Americans Use Social Media.
Here's an excerpt:
Two-thirds of online adults (66%) use social media platforms such as Facebook, Twitter, MySpace or LinkedIn. These internet users say that connections with family members and friends (both new and old) are a primary consideration in their adoption of social media tools. Roughly two thirds of social media users say that staying in touch with current friends and family members is a major reason they use these sites, while half say that connecting with old friends they've lost touch with is a major reason behind their use of these technologies.