Found this good article on social networking and social marketing.
Excerpt from website: Within the last 3 years Social Networks have taken the internet by storm. Startups have created enormous communities bringing millions of people together through user-friendly interfaces and applications. User's can create profiles and share pictures, music, friends, comments, personal information, favorite sites, and much more. Social Networks like Myspace.com, and Facebook.com have grown at exponential rates and continue to grow. In the past 10 years traditional forms of advertising have become a little more obsolete, and social networks have become an alternative route for reaching the masses. Large advertising agencies have turned to social networks as new marketing outlets because they have the ability to reach a large user base.
Read full Article - Top 10 Social Bookmarking Sites to Drive Traffic