From Dave Winer at Scripting News:
Seeding users with test units of products. Or taking 100 bloggers from some random city to a new movie a week before it comes out and letting them write about it.
I think that would make Tumblr a news outlet or at least a review site. Maybe that would work. What do I know. But I don't think the problem is that we have too few armatures publishing opinions. Medium is making a good go of it with that model and it's not exactly blowing the doors off of the place. I think the problem is that to be successful with ads you need to be an advertising company that owns the entire stack – like Facebook or Google.
Tumblr ads were low quality and poorly targeted. It was a makeup shotgun. For the advertisers that only have 4/5ths of a second to make an impression. I don't think that works now.