But comparing Tumblr to WordPress is like comparing apples and oranges. They are completely different things.
WordPress is a publishing platform. You can host it yourself or WordPress it will host it for you. And yes, some people use Tumblr in this use case.
But the vast majority of the Tumblr engagement (traffic, page views, liking, reblogs, follows, etc), is on the Tumblr Dashboard which is their unique & native version of a social newsfeed. The Tumblr Dashboard is where you follow other Tumblr users and traffic inside the Tumblr Dashboard far exceeds (understatement) traffic to the aggregate page views to Tumblr powered sites.