• by Joe Mandese  @mp_joemandese, January 24, 2013

The volume of biddable media inventory being traded in the RTB marketplace doubled during 2012, according to the latest installment of the RTB Marketplace Tracking Report being presented by Accordant Media Co-Founder & CEO Art Muldoon at OMMA RTB as I’m writing this -- in real-time. That’s right -- Muldoon is presenting this exclusively at OMMA RTB, and his other big highlights show that the RTB Marketplace “cooled” a little at year-end, with fourth-quarter 2012 impressions expanding only 61% over the same quarter in 2011.

While desktop browser-based impressions -- display, video, search, email, etc. -- continue to dominate the supply of biddable media impressions, Muldoon said “mobile and social RTB continues to show promise,” especially among performance-oriented CPA (cost-per-action) advertisers.

“On average, mobile inventory performed nearly 80% as well as traditional display,” Muldoon noted.

Focusing on some of the qualitative insights, Muldoon noted that despite the ad industry’s cry for greater transparency -- especially on the subject of ad “viewability” -- there has been little change in the amount of viewable ad impressions data over the past year. Muldoon suggested that might be one of the factors limiting the growth in the RTB marketplace.

Quantitatively, the RTB Marketplace is representing more publishers than ever before, but most of the action is being concentrated among the top 100 sites.

During 2012, the number of sites participating in the RTB marketplace grew from 237,000 in the first quarter to “nearly 500,000” in the fourth quarter.

However, Muldoon noted that the percentage of impressions being traded among the top 100 sites actually grew from 35% in the first quarter to 47% in the fourth quarter. It wasn’t clear from Muldoon whether seasonality played a role in the higher share going to the top 100 during the fourth quarter, but he did say there’s still significant value among the “long tail,” but that “you need a disciplined approach to find that.”