A new study from Nielsen/McKinsey's NM Incite group on Facebook friending:
"NM Incite’s ‘State of Social Media Survey’ is based on a representative sample of 1,865 adult (18+) social media users who were recruited from the Nielsen Online Panel to take an online survey. “Social media user” is defined as participating, talking, and networking online through various platforms to share information and resources. This includes Internet forums, blogs, Facebook, Twitter, video sharing, consumer rating and other social networking websites. The survey fielded from March 31 to April 14th."