Their front page does not explain what the hell it actually is, so all users that join them join because existing users already know what the site actually does and ask friends to join, which of course restricts new users to being close to their existing demographics.
Edit: While i typed this the teeny tiny "About" link at the bottom of the page disappeared. (?!)
Edit2: After browsing around on their about pages, they have a lot of breathless promises of virtually world peace caused by using Pinterest, but little concrete information of what the site is actually useful for. They could do with grabbing some people off the street and pitching them various explanations to find some where the person actually understands the site afterwards.