The space continues to evolve to meet advancing needs. Take a look at what's driving these changes and what this means for marketers.

While no one can predict the future of the internet, it's universally acknowledged that advertising budgets traditionally allocated to TV, newspapers and magazines have been steadily moving online, and it's hard to find anyone who will argue that the trend will slow down in the coming years.

So, why then would anyone suggest that the current crop of ad networks might have seen their best days?

One reason is the very prominent dumping they took from ESPN, specifically at the hands of Eric Johnson, ESPN's executive VP, multimedia sales, who adamantly declined to comment for this article other than to say, "It no longer suits our business needs to work with ad networks."

Another might be the hits many ad networks have been taking in private conversations among advertisers and publishers, based not only on a worrisome lack of transparency in online ad networks, but on irritating issues of channel conflict and growing interest among advertisers in developing online campaigns that seem to demand more than the current generation of ad networks can deliver.

Simply put, advertisers have long been concerned about where their network ads are running, and only recently have begun to receive enough data to find out. Less tractable has been the problem of so many ad networks buying so much inventory on so many of the same websites. The resulting channel conflict (less politely known as "cannibalism") is what presumably drove ESPN recently to change course away from offering its excess inventory to online advertising networks.

On a more positive note, the growing sophistication of audiences and of advertisers themselves -- with respect to online opportunities -- are now creating the climate in which advertisers can think about moving beyond the capabilities of most established horizontal ad networks.

But to say that online advertising is evolving is not to say that "old school" business models have lost their value, or that existing ad networks are not capable of changing to meet these advancing needs.