Advertising on social media sites will increase to 12.1 percent of all online ad spending in the US by 2012, a study from eMarketer projects.

If so, that percentage would be nearly double the 6.3 percent share reported in 2009.

Overall online ad spending is projected to increase to $3.08 billion this year and $3.93 billion in 2012 compared to $1.43 billion in 2009 and $1.99 in 2010.

The big driver behind the increase is Facebook, eMarketer says.

“Major marketers are integrating social media into their overall marketing programs,” said Debra Aho Williamson, an eMarketer principal analyst who is the author of the report “Worldwide Social Network Ad Spending: 2011 Outlook.” “As they do so, advertising in social destinations is becoming a more logical part of their plan.”

Worldwide, social network ad spending will represent 10.2 percent of spending in 2012, up from 8.7 percent in 2011, 5.6 percent in 2010 and 4.3 percent in 2009.

Ad spending overall will hit $8.09 billion worldwide next year and $5.97 billion this year.

“The skepticism of a few years ago has faded; large brands are allocating more marketing budget to social media than ever before, and their social network ad spending is also rising,” Williamson said. “Two categories of advertisers are emerging: major brand marketers that increase budgets gradually, and performance advertisers that spend heavily and bring extensive search marketing expertise.”

For more about the report, read here.

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