The online TV services of the four major U.S. networks, ABC, CBS, NBC and Fox, along with Hulu, accounted for a combined 53 percent of the ad-supported online TV market that generated $448 million in revenues in 2008, according to a new report by Screen Digest.

The remaining share of revenues was made up of the online video services of major sports leagues, video services from traditional online portals, and direct services from other major channel groups and content owners.

The worldwide TV advertising market is expected to see sluggish growth to 2009 but will see a rebound, mainly online, from 2010 to 2012 as TV audiences fragment and online advertising continues to grab a bigger share of marketing budgets, according to Screen Digest.