Tumblr is launching its mobile ads today, marking the first time the 6-year-old startup is placing ads that look and feel like regular posts on the blog network.

The mobile ads--launching today with advertisers like GE, Warner Bros. and ABC--are the first to appear in user streams where until now Tumbr users have just seen content posted by people they follow. Tumblr's other ads had been constrained to the less visible real estate of the "Radar" post that appears on users' dashboards, which alternates between paid promotions and images curated by Tumblr staff, and "spotlight" placements on pages that highlight top blogs in categories from sports to architecture.

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.

"This mobile advertising opportunity is native to how our consumers experience content on our apps; as a continuous stream," said Tumblr head of sales Lee Brown, in a statement.

Among the first advertisers to try Tumblr's mobile ads are ABC Entertainment and ABC Family, GE, Pepsi and Warner Bros., which will debut teasers for the summer movies "The Great Gatsby" and "The Hangover Part III" this week.

It's part of an ongoing about-face for a company that once publicly eschewed advertising, when its founder David Karp told the Los Angeles Times in 2010 that "it really turns our stomachs." He had softened a year ago when he announced Tumblr's first ad -- the "Radar" unit -- at Ad Age 's Digital Conference in 2012, but stated that the company was seeking to work with creative brands.

Aside from advertising, the company has experimented with allowing users to pay to expose their posts to a wider audience on Tumbr.

Its first major brand advertising campaign was for the 2012 UEFA European Championship with Adidas, which already had a Tumblr page populated with the animated GIFs that the platform is famous for.

Last month, Mr. Brown, said that he expected the company to reach profitability this year and that the average advertising purchase on the platform was "just under six figures."

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.

For the sake of comparison, Twitter didn't roll out mobile ads for its native apps until 2012, two years after it introduced advertising. Facebook likewise announced its first mobile ads only in February 2012.