If Pinterest continues their exponential growth rate, they will pass Tumblr like they're standing still in the next few months: http://i.imgur.com/bmrdx.png

I'd proffer that it doesn't really matter what they're doing different from Tumblr; whatever they're doing is working and organically engaging a very large number of users. Try their service if you want to see what state-of-the-art social onboarding feels like circa 2012.