I'd argue that a large portion of that is ineffective ads. There have been a large number of ads for a free-to-play "choose your own adventure" game, and they are frequently reblogged and mocked. (Might explain why non-rebloggable ads are suddenly showing up in my feed!)

Users already follow tumblr blogs that cater to their interests. Tumblr already knows I follow a Pokémon-themed blog, a How To Train Your Dragon-themed blog, and the official blogs for Instapaper and Apple Music. And Taylor Swift.

Based on those interests, Tumblr should be able to sell ads for, say, the new Pokémon mobile game or Taylor Swift's new concert film (premiering on Apple Music), or whatever. It shouldn't matter whether or not I click or do anything actionable on those ads; what matters is that I see them. It's like television ads in that respect: you're paying to get your message out to a group of people.

So yes, tumblr advertising is currently quite ineffective. But it doesn't have to be.