I was wondering that myself, and I saw three options: 1. Designed and marketed towards women; 2. A core group of early female users influenced the direction of the site and made women more likely to join, seeing relevant content; 3. Something about the format of the site itself made women more likely to want to use it.
I don't think it's #1. From CNN:
A Yale grad with no engineering background, Silbermann worked for Google before launching Pinterest with some friends in late 2009. Real-time text feeds were the rage at the time, and some observers felt that an image-based pinboard was doomed to fail. Nine months later, the site still had less than 10,000 users.
It sounds like it was created in a rather gender-neutral way.
(how do you make inline quotes?)