As a long-time Tumblr user I've always wondered, do they have income? There are no ads on the site, and no premium plans or pricing (other than the paid themes

As a long-time Tumblr user I've always wondered, do they have income? There are no ads on the site, and no premium plans or pricing (other than the paid themes, but I assume that most of that goes to the theme's author) that I can see. How can it last?