Why are MySpace and Facebook so valuable? Because people use them. Personally I can't stand MySpace. I think the layout is poor and it is not a user friendly site. I think part of the reason Facebook took some of their market share is because it is easier to use.
What makes these social networks valuable is that they provide a platform for users to generate and share their own content. The more people use the site and generate content, the more valuable the site becomes.
Lesson: Make your site interactive and allow your visitors to contribute their own content.
How can customers contribute to your website?