Tumblr and Wordpress are blogging websites that allow their users to publish posts. Most users just publish their own personal posts on their own account. But these websites also allow a user to operate as an intermediary where other third-party users can publish posts through the first user's account.
Since Tumblr and Wordpress host the actual site, who has the duty to report child pornography to NCMEC if one of those third-party users submits it: The first user who created the blog, or the host of the site (Tumblr or Wordpress)?
Another way to consider the question is, who is the "internet service provider," the creating user who has administrative powers on his blog where third parties are submitting the offending content, or the corporate host of the entire social network?