Popular social networking sites, including MySpace and Facebook, are changing the human fabric of the Internet and have the potential to pay off big for investors, but -- given their youthful user base -- they are unusually vulnerable to the next 'new new' thing. As quickly as users flock to one trendy Internet site, they can just as quickly move on to another, with no advance warning, according to Wharton faculty and Internet analysts.

MySpace, with 70 million visitors, has become the digital equivalent of hanging out at the mall for today's teens, who load the site with photos, news about music groups and detailed profiles of their likes and dislikes. Other social network sites include Facebook, geared to college students, LinkedIn, aimed at professionals, and Xanga, a blog-based community site. In all, an estimated 300 sites, including smaller ones such as StudyBreakers for high schoolers and Photobucket, a site for posting images, make up the social network universe.

Wharton marketing professor David Bell says the long-term success of these sites will depend on their ability to retain the interest of their members. "There is a fad or a fashion component to all these networks. Some will come and go," says Bell. The classic example, he suggests, is Friendster, which burst onto the Internet in 2003 and soon had 20 million visitors. Late last year, it slipped below a million after MySpace and other sites with better music and video capability lured Friendster users away. "A lot of the [success] is serendipitous. These things can have exponential growth. Then, if another community shows up that has better functionality in some way, there can be a mass migration."

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