In 2005, U.S. internet advertising spending will pass the $10 billion mark for the first time, reaching $12.9 billion - more than double 2002's $6.0 billion - according to eMarketer projections. In 2009, online adspend is expected to increase by another $10 billion, reaching $22.3 billion. "Much of the increase is coming at the expense of other, traditional media channels," says David Hallerman, Senior Analyst at eMarketer.

Four trends explain the expected growth: Larger ad budgets overall, shifts to online in ad budgets; more companies' resorting to online as part of their campaigns; increased prices for display and rich media as well as direct response, such as paid search.

According to an InsightExpress survey, 74 percent of media planning and buying executives reported an increased demand for internet ad inventory in late 2004 - 23 points higher than cable TV (the second most in-demand medium).

Hallerman notes, however, that despite rapid growth, online advertising hasn't kept up with audience growth: though adspend rose 32.5 percent in 2004, spending per user rose 27.9 percent.