Pinterest seems to be growing exponentially. We found this interesting material on RJMetrics which explains things with more data & analysis. Here's the summary:

• Pinterest is retaining and engaging users as much as 2-3x as efficiently as Twitter was at a similar time in its history.
• Pins link to a tremendously large universe of sites. Etsy is the most popular source of pin content, but it only represents about 3% of pins.
• Over 80% of pins are re-pins, demonstrating the tremendous virality at work in the Pinterest community. To contrast, a study done at a similar time in Twitter’s history showed that only about 1.4% of tweets were retweets.
• The quality of the average new user (as defined by their level of engagement and likelihood to remain active) is high but declining. Users who have joined in recent months are 2-3x less active during their first month than the users that came before them.