• by Joe Mandese  @mp_joemandese, June 7, 2010

Worldwide online ad spending will rise 12.4% to $61 billion in 2010, according to a revised forecast released this afternoon by Interpublic’s Magna Global unit.

Paid search will expand at an even faster rate, rising 16.5% to $29.8 billion, and accounting for nearly half of global online ad spending in 2010, according to Magna’s estimates.

“Google remains by far the global leader in search, although a handful of other suppliers of search advertising are dominant in certain countries,” Magna noted in its report. “Unfortunately for Google, China and Russia – the fastest growing large markets for online advertising – pose a challenge for foreign-based search engines.”

The agency said that the “enormous influence – and profitability” of paid online search advertising has led some governments to become more involved in the marketplace in recent years, but it said that “such actions are unlikely to constrain the medium’s growth.”

All other forms of online ad spending will grow at about half the rate of search, expanding 8.7% to $31.2 billion.

“ Other online advertising is much more diffused, with a handful of global portals, such as Yahoo and Microsoft, and many regionally strong publishers (often associated with print publications) capturing most of that sector’s revenue,” Magna wrote. “Although they provide an important constraint on inflationary conditions, advertising networks retain their importance to advertisers given their ability to aggregate and monetize vast quantities of inventory in an inexpensive manner. Social networking sites such as Facebook capture a large and growing share of audience time. While advertising is becoming increasingly important for social networks – and is undoubtedly growing much more rapidly than the rest of online advertising – they generally punch ‘below weight’ given the premium pricing that conventional content publishers can extract from their inventory.

Looking ahead, Magna projected that only ad spending trends should remain robust over the next give years, rising 11.7% in 2011, and growing an an average annual rate of 11.0% through 2015.