Real-time bidding (RTB) will account for 19% of all U.S. digital display ad spending -- or $3.34 billion -- this year, according to eMarketer. The revised numbers released in June suggest a change from 72.7% growth to $3.32 billion. The firm now expects advertisers to spend $8.69 billion on RTB ads by 2017, up from the $8.51 billion previously forecast.