Dive summary: 

• According to those in the ad industry, a Tumblr ad exchange is inevitable, even if it alienates the anti-ad user base. 
• Since the $1.1 billion acquisition of Tumblr, Yahoo CEO Marissa Mayer has hinted at an ad exchange for the social blogging site. 
• The advertising will likely be "native" in nature so as to minimize user backlash, but after the success of Facebook and Twitter's exchanges, Tumblr is likely to follow suit. 

From the article: 

"For fraud-weary marketers, part of the allure of site-specific ad exchanges—rather than ones with hundreds of sometimes-random URL destinations—may point to the protective nature of the so-called walled garden for the Facebooks, Twitters and Tumblrs of the world. Steinberg, who helps run TWC's self-contained WeatherFX exchange, said, 'What we are seeing is a lot of marketers coming to us, saying, 'We only want to run on a handful of properties. We are done with buying across the entire Web.'"