Does Tumblr host a lot of pictures of naked people? Yep. But they've got an ad plan that deals with that.
If you write about Tumblr as a business, you are required to note that Tumblr has a lot of porn.
How much porn? You’ll have to make something up, because the only people who know much porn the blogging service hosts work at the blogging service, and they don’t offer up a number.
But let’s stipulate, for argument’s sake, that there is indeed a lot of porn on Tumblr — in fact, the company’s terms of service make a point of saying it’s ok with “not suitable for work” stuff.
Which means there are a lot of pages on Tumblr that advertisers won’t go near. Like “Girls in Yoga Pants“, where the image at the top of this post came from (yes, that’s a tame one).