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.…