By some measures, this is expected to grow to as much as 60% in 2008. This is remarkable - what could possibly be driving this kind of growth in market share?
I believe the dramatic expansion is being driven by a number of advances in key areas. Firstly, networks offer marketers powerful technology that can pick out the best possible customers from the myriad of placement options provided by today's media landscape.
There are some amazing, cutting-edge technologies in production by the world's hardware manufacturers, which test the boundaries of what's achievable.
The leading networks can serve high volumes of ads, utilise vast stores of data and layer on targeting technologies that seek out large numbers of customers predisposed to a deep relationship with an advertiser's brand.
Secondly, many networks are demonstrating a more careful buying policy, focusing on larger, better-known, higher-quality publishers. While many networks offer access to the long tail, more reliable successes are being driven by concentrating on delivering results from the shoulder and head, environments where a brand can be shown safely.
Publishers are also receiving better-quality, more targeted ads on their sites, and are seeing their yield steadily increase. Industry bodies, notably IASH, are taking a real leadership role in educating the buyer in how to achieve more from their advertising campaigns.
Finally, the very definition of "network effect", where a service becomes more valuable the more people participate in it, suggests we're going to see even more of ad networks in the future, providing they keep delivering what their customers want.
Colin Petrie-Norris, managing director, international, Specific Media
As consumers' internet use fragments across blogs, social networks and a myriad of niche sites, buying media from individual publishers is a less than attractive option for advertisers.
This is where the UK's 85 ad networks come in: firms that take the pages that publishers can't shift directly and sell that inventory on to advertisers at cheaper rates. Variations on the theme include third-party sales houses such as Ad2One and Woot!Media, and ad exchanges such as Right Media, which sell media space through automated dealing systems.
In theory, the system benefits all parties. Publishers raise revenue, while advertisers gain access to huge audiences through the pooled networks, which can reach up to 85% of online users.
However, the industry's reputation has been damaged by some operators who paid little attention to where ads were placed - last July, ads from brands such as eBay and Orange appeared on an inappropriate fighting site.
Industry body the Internet Advertising Sales House has shown it is serious about restoring advertisers' confidence by developing a code of conduct and suspending 24/7 Real Media for failing to take part in an audit. The network is still waiting for its re-entry to IASH to be approved.
Advertisers also want to know their campaigns will deliver results. Here, networks are boosting click-throughs and sales by using sophisticated techniques such as retargeting, where consumers are targeted at different stages as they move through the network.
Network heads are confident the reputable players will survive the economic downturn. As Ian Woolley, OTPmedia managing director, says: "The networks that focus on performance and building strong relationships with publishers will come out singing."
But as Google and Yahoo take ever-more share of the display market, the smaller players must build niche offerings to compete. For example, Oridian UK, a Ybrant Digital Network, launched a foreign channel in June targeting non-UK nationals.
Unless the other independents develop similar specialist offerings, it will become harder for the smaller networks to justify their existence.
Harriet Dennys, features editor, Media Week