Then that company would be an underdog in the pinboard business, with lost referrals (Etsy, Amazon) or a distraction from the core focus (WordPress, Blogger), and have to spend several years building a community of users who think to go to the new site to pin things. And other sites would presumably also block this upstart.

If pinning were just a technology feature Tumblr seems the closest to being able to duplicate it (their archive view seems vaguely close to a pinboard, minus the interactivity), but they would also have to provide users with a better alternative.