Tumblr brings sponsored posts to the web
sponsored tumblr

Starting today Tumblr's sponsored posts are coming to your dashboard on the web, and they look just like the ones in Tumblr's mobile apps that launched a year ago.

While these are indeed advertisements, they do fit in pretty well, as most of the time it isn't obvious that they are sponsored until you see the icon at the top right of the post box. Advertisers seem to understand how to appeal to Tumblr's users.