—The pitch: So what have Walrath and Yahoo been saying to convince agencies to join them? Agencies have realized that the ad exchanges, which offer auction-based, automatic targeting for ads across thousands of websites, still don’t represent much of a threat to traditional media buying. The idea is that exchanges can extend the advertisers’ reach on the web, supplementing the usual functions of media buying and promising greater efficiencies. “The real benefit here to WPP’s clients is that they can be more precise in what they target and, obviously, create more return for marketers’ ad spending.”

—Learning curve: Publishers may still be concerned that exchanges and ad networks treat ad space like a commodity, but advertisers don’t seem to mind. “There is still a lot of confusion about ad exchanges and ad networks, mostly on the part of publishers, but really, it’s akin to the differences between the New York Stock Exchange and Merrill Lynch. We’re starting to see, and the partnership with WPP represents this, companies are embracing new ways of managing their digital ad spending. The only concerns advertisers have - which stem from publishers’ wariness - is whether this will ever be a liquid enough market for this to be really beneficial, will it ever get to a large enough scale. But with 6 billion ad impressions a day being traded among 5,000 participants in the marketplace, I can say that we’ve reached that scale. And this is where the tipping point comes.”