Many lines are involved.

On Pinterest, every pin ties back to an external link. We used RJMetrics to extract the top-level domain of those links for the pins in our sample. What we found was a pretty tremendous long-tail effect. In our sample of about a million pins, over 100,000 distinct source domains existed.

Top pin sources: Etsy; Google Image Search (so actually other sites); Flickr; Tumblr (more third-party stuff?). After the top 5, no domain represents more than 1% of pins.

Basically, Pinterest is the long tail turned into a website.