In 2012, search engine and online display are still expected to be the two largest contributors, generating more than $20 billion and $10 billion, respectively, according to Forrester.

However, online video and emerging channels are growing strongly, which are expected to contribute more than $15 billion in 2012 altogether, while e-mail remains relatively flat throughout the years, according to the World Digital Media Trends 2008 report, released by the World Association of Newspapers.