A quarter of all display-ad spending in the U.S. will occur via real-time auctions for ad impressions by 2015, according to research firm eMarketer's first forecast of the growing ad-buying trend. But eMarketer is cautioning that its growth numbers are a bit more "conservative" than some other estimates, owing in part to uncertainly around how quickly mobile and online-video-ad spending will ramp up as well as the extent to which Facebook will open up more of its ad inventory to real-time bidding.

Still, eMarketer is estimating that RTB ad spending in the U.S. will grow 72% next year to $3.4 billion, up from $1.9 billion this year. The RTB market will grow considerably slower in 2014 -- 36 % year over year -- as the market begins to mature. By the end of 2016, display-ad spending through real-time bidding should top $7 billion to make up 28% of total display-ad buying, eMarketer estimates. Online-video and mobile ads bought through real-time bidding are included in the estimates.

Growth estimates for nascent markets such as RTB are essentially educated guesses -- and this forecast is no different. But such estimates can still be useful in giving marketers and media companies at least a directional look at the shifting landscape of digital advertising.

Emarketer comes up with its estimates by combining analysis of other research-firm estimates, such as a recent one from Forrester, with information it gathers through its own surveys and interviews with media buyers, marketers, and media salespeople.

But with such a dynamic market, eMarketer acknowledges that these numbers will likely shift. The evolution of the Facebook exchange -- which allows advertisers to use real-time bidding to purchase ads on the right rail of Facebook pages -- which just launched this year, will be one major influencer.

"Depending on how quickly Facebook scales the Facebook exchange, you could see a variety of different growth scenarios," eMarketer spokesman Clark Fredricksen said. Publishers' willingness to open up more of their online-ad inventory to real-time-bidding through mechanisms such as private ad exchanges is also a trend to watch.