Online advertising spend from companies in the media and entertainment, financial services, and travel and automotive categories will account for the majority of online advertising spending within five years.

Online advertising spend in those four categories will total $11.5 billion in 2011, according to a new JupiterResearch report, "US Online Category Advertising Forecast, 2006 to 2011." That amount will account for 57 percent of all online ad spending that year. "Media & entertainment and financial services advertisers benefit from business models that work well online," said Emily Riley, JupiterResearch analyst and lead author of the report.

In 2011, automotive and travel advertisers will be the third and fourth largest contributors, respectively, to online spending, following financial services in second place, according to Jupiter. The largest online advertiser category will be media & entertainment, and it will also account for over one quarter of search spending overall.

"Audience fragmentation and ecommerce patterns will continue to drive brand and direct response advertisers to shift off-line budgets online," said David Schatsky, president of JupiterKagan. "With consumers spending as much time going online as watching television - a median of 14 hours per week - the shifting of budgets to online advertising is inevitable."