Fwiw, pinterest didn't start with categories. A few weeks ago, I did some blog archeology looking for early mentions of Pinterest and examined the screenshots,

Fwiw, pinterest didn't start with categories. A few weeks ago, I did some blog archeology looking for early mentions of Pinterest and examined the screenshots, and announcements on the company blog. They started out with tags on each pin. I don't doubt that they looking at the advertising value of certain keywords, but I suspect they probably analyzed tags and board names to inform the choice of categories.