As far as I know there is no industry group for still-image photographers anything like the MPAA or RIAA. All three of those sites are and were filled with substantial original content, so they can claim that illegal use is not their primary drive. I don't know if Pinterest can successfully argue the same thing.
It also should be noted that YouTube spent a great deal of money on settlements and arrangements with RIAA & MPAA members, content networks and others to survive.
"As far as I know there is no industry group for still-image photographers anything like the MPAA or RIAA."
The American Society of Media Photographers, Graphic Artists Guild, the Picture Archive Council of America, the North American Nature Photography Association, Professional Photographers of America. They do not have the deep pockets or political clout of MPAA or RIAA but they are large industry groups.
I think someone else mentioned a class action suite against Google for copyright infringement by photographers. The lawsuit started with scanning and displaying images from the Google Library Project but includes infringement claims for images.google.com, etc.
I've seen warez sites with exactly the same disclaimer. It didn't work for them either.