Pew Internet & American Life Project's December 2008 tracking survey revealed some insight into users' online social networking behavior. There are few surprises, but some important points to note. First, the use of social networks has quadrupled since 2005. However, that's largely due to users discovering these sites for the first time. Facebook wasn't founded until 2004 and then it was for college students only.
The bulk of users are young adults, but the number of those using these networks is staggering. A full 75% of online adults ages 18-24 are using social networks, 57% of those 25-34 and 30% of those 35-44, according to the study.
Networks are being used for mostly personal reasons, with 89% reporting they use the sites to "Stay in touch with friends," and 49% claiming "Make new friends." But there are certainly business users, with 28% claiming "Make new business or professional contacts," and 28% reporting, "Promote yourself or your work."
Another interesting note is the use of multiple profiles. A full 51% reported having two or more online profiles. In another interesting note, just 36% of users reported having an open, public-facing profile. The rest are viewable by invitation only.
And if you're interested in marketing through social networks, this report suggests you should target students and minorities. According to the study, 68% of full-time students and 71% of part-time students have a social profile(s), compared to just 28% of non-student adults. Also, 31% of white adults are using the sites, compared to 43% of African-Americans and 48% of Hispanics. You can read more of the report here.