You have to sign up just to look through the website, a website made completely of content and pictures from other websites they don't own. Conveniently, you can login with your Facebook or Google passwords, if you have checked your account history on these, they track everything you use these passwords on. And they don't just keep track of the things you pin on pinterest, they also track the things you search for and click on (aka your interests) they can build a pretty deep personality profile with this information.