The head of media buying group ZenithOptimedia is expected to give a rosy outlook on digital ad spending at the UBS annual media conference in New York today.

Chief exec Steve King will tell the gathering that he expects the web to become the world’s third largest ad medium by 2010, behind only newspapers and TV, according to The Times.

He will apparently predict three milestones for the online ad industry …

… “to overtake radio advertising in 2008; to attain a double-digit share of global advertising in 2009; and to overtake magazine advertising in 2010, with 11.5% of total ad spend.”

In truth, no one seems sure about how quickly brands will actually shift budget onto digital platforms over that timeframe. What we do know is that during 2007, ZenithOptimedia has been steadily revising its online ad projections upwards.

In July, it said the web’s share of global ad revenues would increase from 6.1% in 2006 to 9.4% in 2009, having previously predicted it would reach 8.6%. It has since increased this again, to 9.5%, according to Paidcontent. And now we’re into double figures…

King is also expected to announce that the UK will be one of only four countries where 20% of ad revenues are web-based by 2010. The others will be Denmark, Norway and Sweden – the only other countries where that figure is currently 15% or more.

In the UK, proportional online ad spending is the highest in the world, according to the IAB, and is also already ahead of newspaper ad revenues. Next stop radio.

Related research:

Online Ad Networks’ Buyers’ Guide

Search Engine Marketing Buyers’ Guide

Internet Stats Compendium

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