Fundamentally, market forces are dictating these changes.

"The barriers to entry are so non-existent that anybody can start an ad network, just by aggregating inventory on more than one site and having more than one advertiser," says Undertone's Cassidy. "You'll see lots more networks launch."

But new players entering this market generally don't want to try and build horizontal networks, a space already dominated by strong leaders, in which smaller players simply don't find sufficient market opportunities. That's one reason vertical networks are so hot: newcomers can see that vertical networks' value proposition to advertisers ("we can target exactly the customer you want") applies to a large number of interesting niches where leadership can still be forged.