Pinterest always makes me think of Napster. About 15 years ago it was perfectly "normal" to upload and share copyrighted work (music) through Napster. Everyone was using it. Even my bosses wife had Napster on her PC at work to download music.
I just had a look at what Wikipedia had to say about Napster and the "Legal challenges", "Lawsuit" and "Shutdown" parts are quite interesting.
Napster's problem was the sharing of copyrighted material and not being able to prevent copyright violations 100%. Pinterest is doing the same thing. Maybe it's only a matter of time before Pinterest faces legal problems.