ZenithOptimedia's revision takes anticipated 2008 U.S. advertising growth down by nearly half a point, from 4.1% in its last forecast in December 2007, to 3.7% now.

While ZenithOptimedia is actually upping its outlook for the rest of the world (to 11.1% from 10.9%), its outlook for total worldwide ad spending was downgraded from a 6.7% rate of growth in is last outlook to 6.5% now.

Among the major media, the Internet continues to shine, despite overall economic uncertainties. "Once again, we have substantially increased our forecasts for Internet advertising as it has continued to exceed our expectations," ZenithOptimedia says in its latest report, which projects the Internet will take nearly 10% of the global advertising marketplace in 2008. ZenithOptimedia boosted the Internet's share of worldwide ad spending to 9.7% for 2008, up from 9.4% in its December forecast. Next year, it is projected to have an 11.1% share of global ad spending, up from ZenithOptimedia's previous 2009 forecast of 10.4%.

"Online video and local search are now generating substantial new revenues," the agency's report notes, adding, "in the slightly longer term we expect behavioral targeting on social networking sites to provide fruitful new opportunities to advertisers."

ZenithOptimedia said much of online ad spending growth is coming at the expense of some traditional media, especially print editions of newspapers.

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