After five years of ad-free blogging, Tumblr has announced its plan to introduce paid advertising at Ad Age’s Digital Conference yesterday.

Founder and CEO David Karp revealed that as of May 2nd, advertisers will be able to buy ad units on the “Radar” section of Tumblr’s dashboard — which is used to highlight notable and popular posts from across the network.

Previously Radar has been used to promote content from brands — such as The Cartoon Network. Currently, this section receives approximately 120 million impressions a day.

In the past, Karp has been vocal about not wanting to integrate advertising into the microblogging platform. In 2010, he told the Los Angeles Times, “We’re pretty opposed to advertising. It really turns our stomachs.”

Obviously things have changed as Tumblr continues to grow at an impressive rate. The site now has more than 52 million Tumblr blogs, which have resulted in more than 21 billion total posts, 53 million of which were uploaded on April 18th.

Tumblr first toyed with the idea of monetization in February when it introduced Highlights, which enables members to promote posts with a $1 label that will appear in your followers’ streams.

It’s still unclear how much the Radar ads will cost and who Tumblr’s launch partners are. While it’s possible that this might be seen as the underdog selling out, it seems like a logical next step. Brands could certainly benefit from the ad unit, and as long as Tumblr doesn’t get carried away (a la Facebook), it should be well received by its user base as well.

[Via: Ad Age, Image credit: jessaax]