Pinterest, the social network where users “pin†photos to virtual pin boards, has become the third most popular with Web users behind Facebook and Twitter, a report from marketing company Experian says. Like all social networks, Pinterest is dogged by questions of how to make money but Experian’s data suggests that it may not be difficult. Ads for consumer goods and retail are the most successful kinds on Facebook, Experian says, categories that are a natural fit with the photos of home décor and fashion items most pinned by Pinterest users.