Tumblr this morning is rolling a new mobile-only ad unit called "Tumblr Sponsored Apps" which will allow app publishers to target the social blogging service's largely Millennial and heavily mobile audience with creative posts designed to drive users to the iTunes App Store and Google Play to download new applications. App install ads, as these units are commonly called, are already a big business for rival social networks like Facebook, where mobile ads now account for 66% of its total ad...
