The first participating advertisers are ABC, GE, Pepsi and Warner Brothers. AdAge described how the ads will work:

“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.”

Tumblr tiptoed into ads last year with Radar, an ad unit that appears on a user’s dashboard. Tumblr founder David Karp said at the paidContent Live conference last week that the Radar initiative focused on “the type of advertising that creates intent. It gives room for the most creative advertisers to create their best work.” Tumblr is not yet profitable, but head of sales Lee Brown told Bloomberg last month, “We expect that the monetization will lead us to profitability this year.”