It's interesting that this question keeps getting mod'd up and then mod'd back down again. Elephant in the room? For as successful as Tumblr's growth has been, my guess that it's in large part due to the platform distributing a bazillion free porn images and videos. I would be curious to know how much of their traffic is actually porn. My sense is it could be > 80%.
Fwiw, I have nothing against porn. I think it is part of what powers the internet. But I think it's disingenuous to tout the success of a site when you're giving away something for free (much like free music or videos) virtually everybody wants.
