I also don't buy the copyright infringement argument. Pinterest is a bookmarking service much like Delicious. It no more infringes than the thumbnail previews in Google Images or Facebook posts.
Pinterest doesn't just keep thumbnails, it keeps large versions of images on their servers. There are two clicks to get to the page where the pinner found the image.
I can imagine content creators being unhappy that their image is posted to some blog, and then pinned without their name. People seeing the pin may (but probably don't[1]) click twice to get to the blog, but how many of those people then click again to get to the creator of the content?
To me, the artistic pinboards are gently problematic. The product pinboards are less troubling, because I guess they drive some traffic to the selling sites.
[1] I have no data for this. I realise I might be wrong.
I don't use Tumblr, but my understanding is that a user has to upload an image for Tumblr to host it. The DMCA safe harbor provisions apply in such cases. Pinterest itself downloads images from pinned items, which probably makes them liable for the infringement.
I can imagine content creators being unhappy that their image is posted to some blog, and then pinned without their name. People seeing the pin may (but probably don't[1]) click twice to get to the blog, but how many of those people then click again to get to the creator of the content?
To me, the artistic pinboards are gently problematic. The product pinboards are less troubling, because I guess they drive some traffic to the selling sites.
[1] I have no data for this. I realise I might be wrong.