It seems that some people may want to know how to do this so check out the blog post if you have a few minutes.
Pinterest‘s social integration is very well-done, and has contributed to the fast growth of the sharing service. However, you may not want to broadcast everything you do on Pinterest to your Facebook friends. Similarly, you might not be interested in every single thing your Facebook acquaintances pin.
We can help you out. Our simple walkthrough gallery demonstrates how to stop Pinterest from posting on Facebook, how to tailor your settings so you only share Pinterest content with certain people, and how to prevent others’ Pinterest-related posts from cluttering up your news feed.
Continued at: http://mashable.com/2012/05/09/hide-pinterest-facebook/
FYI, you need to click on the blog link to view the slideshow.