It works very simply: Every now and then you’ll see posts from our partners as you scroll through your mobile Dashboard.
The ad within the stream is the new standard.
Interesting that they go mobile first. AdAge:
Users of Tumblr’s iOS and Android apps will see up to four ads per day, and they’ll be differentiated with a dollar-sign icon with beams shooting out of it, just as they are in the two existing placements. A Tumblr spokesperson said these ads will ultimately migrate to desktop computers but offered no timetable.
[..]
The decision to roll out ads in mobile stream first also underscores just how quickly the mobile ad market is growing. U.S. mobile advertising will be a $7.29 billion industry in 2013, according to reported issued by eMarketer this month, an upward revision of $100 million from December.
Facebook and Twitter only began showing mobile ads in 2012.