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To my knowledge, Pinterest is a very high quality linking source. Having several links back and forth should mean that you have an equal amount of images, products and/or pages. No matter how good the source is, when you over use something it doesn't look natural. From what I understand, Penguin 3.0 really targeted linking "neighborhoods". This means that you could see an effect because of link associations that your linking sources have. That could be Pinterest, but my gut tells me that it is probably some other source. These social media sites are pretty safe and usually have ways to prevent bad associations. Have you used Open Site Explorer to compare what you have in GWT? I would just take a look and see what the quality of your links are and if there is any low hanging fruit you should disavow. You want to find high quality linking sources, these are the safest. I would try to eliminate links from low ranking domains with not a lot of root links behind them. You might be suffering the effects of someone else's bad links. Have you received any manual notices in GWT?