SeekingAlpha seems to be doing quite well and their content is all driven by paid user submissions. I've never submitted content, but as a user I love it. Very high quality articles.
It's also very valuable traffic to advertisers so they can pay out attractive rates to writers while also making good money being the middle man.
If Pinterest, Twitter or Tumblr were to adopt this model, it probably wouldn't be very effective because their traffic is (essentially) worthless on a micro scale.
