Knowledge@Wharton has an interview with Joe Kraus, director of product management at Google, in which he highlights the importance of social interaction on the Web:
"So, the killer apps that have really worked on the web have always been about connecting people to one another. So, whether it is instant messaging and e-mail as communications to connect people to one another, whether it's photo-sharing as a way to connect people to one another through photos, or blogging as a way to connect people to one another through the words, people have always been social and the killer apps that have really succeeded on the web have always been social."