Pinterest is still a fairly young company but last year alone, 2012, Pinterest grew from 9 million to 40 million users.
While the user demographics are going to vary slightly depending on geographic location, it seems that no matter where you look, females are going to make up about 80% of users.
Also, the two largest age brackets of Pinterest users are 35-49 year olds and 25-34 year olds. Additionally, over 75% of all Pinterest users are white, with Hispanics making up most of the remaining 25%. Here is a comparison of the main intrests between users in the United States versus the United Kingdom.
Keep an eye out for my next post about the growth of Pinterest as its own brand!