NEW YORK (TheStreet) -- Television's 50-year domination of global media is about to end.

Spending on digital advertising worldwide will overtake purchases of television ads by 2017, according to Magna Global, the international advertising monitor. That's two years earlier than Magna Global forecast a year ago. Next year, digital advertising will make up about 30% of the global pie. It's expected to pass television around the 38% level.

Digital media's ascendancy is bad news for television networks and television station owners even as it underscores the move by CBS (CBS) and Time Warner (TWX) , among others, to create separate online offerings for its premium channels and non-primetime programming. TV advertising in the U.S. is expected to fall 1.4% next year, Magna said, compared with advertising on digital media, which is expected to grow 15.5%.

While television hits a tipping point, ad revenue from digital media grew 17% to $142 billion this year, thanks to mobile campaigns and new social formats, Magna Global said. Social media ad spending will account for one-third of all ad spending next year, according to ZenithOptimedia CEO Steve King.

Unlike in years past when digital's growth was largely fueled by advertisers spending less on print, digital is increasingly taking from television, said Vincent Letang, Magna Global's director of global forecasting, on Monday at the UBS Global Media and Communications Conference in New York. That's a shift that even David Poltrack, the longtime CBS research director, had to acknowledge even as he argued that broadcast TV represents a better use of ad dollar for both total reach and cost-effectiveness.

While that is open to debate, Poltrack cited advertiser resistance to "upfront" TV purchases, time slots bought in bulk months ahead of airing, and the "mercurial" nature of in-season advertising buying as having the greatest drag on TV ad spending. Ad growth in 2015, he said, will be little changed in  2015.

"The ad market never gained any momentm in 2014," said Poltrack, who said advertising at U.S. broadcast-TV networks grew 1% in 2014, well below his forecast for 6% growth made a year ago. "I missed the call for 2014 by a wide margin."

All told, global advertising is expected to rise 4.8% to $536 billion in 2015, a deceleration from this year when worldwide ad spending will have increased 5.5%, said Magna Global.

Advertisers, Letang said, are following consumers' viewing habits. The amount of time the average viewer spent watching TV peaked at 32 hours a week in 2009, and that total has dropped to 27 in 2014. The change, he said, "goes to consumption of nontelevision video consumption." Exactly where that viewing is going will become clearer once Nielsen starts measuring Netflix (NFLX) , he added.

Streaming video rather than by-appointment television watching is changing media consumptin, if it hasn't already, Poltrack added.

"Growth of streaming is the major disruptive force in the media landscape today," he said David Poltrack. And streaming of shows isn't just happening for primtetime programming but outside the evening hours as well. Poltrack argued that broadcasters are well servced by licensing thier content to Netflix as it brings both increased engagement for their programmiongs generally as well as additional revenue.

Television, Poltrack warned all doubters, is hardly going away. It's focus remains on expanding its online offerings.

Television company owners led by CBS CEO Les Moonves and Time Warner CEO Jeff Bewkes have complained that traditional measuring agencies, led by Nielsen, have lagged current consumption patterns. That view was echoed by Martin Sorrell, chief executive of the global advertising agency WPP, who argued that as measuring agencies increase their sample sizes to better include mobile platforms, the results may be more favorable to television content.

"That will force the measurer to become more representative in their sample size -- much more urban and rural," Sorrell said. "It may show TV viewing is not down but being consumed in other forms."

-- Written by Leon Lazaroff in New York

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