ZenithOptimedia forecasts global internet ad spend to grow 28.2 percent in 2007, while ad spend in other media will grow just 3.9 percent.

The internet's share of total ad spend will increase from 5.8 percent in 2006 to 8.6 percent in 2009, according to the report, and is headed for well over 10 percent.

The internet will overtake outdoor this year and radio in 2009

Paid search is currently the largest type of internet advertising, and the gap between search and display is widening. Merrill Lynch recently estimated that search will account for 42 to 43 percent of online advertising spend next year.

However, display includes video ads and other rich media applications high-speed broadband makes possible, and still has a lot of potential for growth. The American Advertising Federation recently found that most leading ad executives expect a large share of their advertising budgets over the next few years, originally slated for broadcast and cable TV advertising, to shift to online video buys.

Classifieds continue to migrate from print to online, as well.

Part of the reason for the exponential growth according to the report is that internet ad spend still isn't up to internet usage. In 2005 consumers in the United States, Japan and the UK (the top three ad markets) spent 21.9 percent of their media time using the internet, yet advertisers in these three markets spent only 6.8 percent of their budgets online-- a consumption-to-spending ratio of more than 3:1.

Read more about  ZenithOptimedia's report.