Each of the top online ad network delivered ads to at least half the US internet audience in March - with leader Platform-A reaching 91 percent - according to comScore, which also notes the emergence of a series of audience-specific niche ad networks, MarketingCharts reports.

Below, the March findings released by comScore:

Top 15 Ad Networks

Among the highlights:

• Platform-A, the AOL ad network that combines Advertising.com, Quigo and Tacoda, served ads to 170 million US Internet users in March, representing 91 percent of the total US online population.
• Advertising.com alone would rank as the top ad network with a reach of 167.5 million Internet users.
• Yahoo Network ranked second with a reach of 160.3 million, followed by Google Ad Network (152.0 million), Specific Media (145.6 million) and ValueClick Networks rounding out the top 5 with 140.1 million.

"Ad networks have become integral components to the online advertising industry over the past several years by helping aggregate audiences for the purposes of delivering impressions to a broad array of Internet users," said Jeff Hackett, comScore VP. "The increasing ability of many ad networks to target and deliver ads either behaviorally or demographically is enabling a more efficient expression of ad dollars and an improved return on advertisers' online marketing investment."

Niche Ad Networks

As ad networks have expanded their reach and influence online, new ad networks have emerged to serve specific demographic and behavioral target segments through both the traditional ad network model and more innovative ad-targeting mechanisms. Their ability to precision-target niche audiences helps advertisers reduce wasted ad impressions.

Among the selected niche ad networks highlighted:

• An example of a behaviorally targeted ad network is Snap Shots Network, which delivers ads to users of Snap.com's Snap Shots. The network reached more than 18 million US Internet users in March.
• Widgetbucks Network delivers contextually relevant ads through a widget, and joined the ad networks ranking in March with a reach of 9.5 million.
• NeoEdge Game Network, which delivers ads through games, had a reach of nearly 1 million.
• Other ad networks on this list target specific audience segments, such as HispanoClick by Batanga (Hispanics), Indieclick (young influencers or "tastemakers") and The Heavy Men's Network (men).

"The reality is that advertisers have different objectives with their online campaigns," Hackett said. "While some are focused on broadening the reach of their brand, others prefer a more focused approach that can deliver a higher frequency. The emergence of these niche ad networks represents an evolution for the online advertising industry that is giving advertisers more precise targeting and better control of their online campaigns."