U.S. marketers will spend $2.74 billion on social marketing ads this year, according to a new eMarketer report. That accounts for 8.75% of the $31.30 billion marketers will spend on online ads in 2011, the research firm says.

The amount marketers are spending on social marketing is rapidly increasing, eMarketer says. By 2013 marketers will increase their social marketing spending 75.5%, to $4.81 billion. That, in turn, will increase the social marketing’s percentage of overall online advertising spending to 11.67%.

“As more companies build out their marketing presence in social networks, they are also increasing their focus on spending ad dollars there,” says Debra Aho Williamson, eMarketer principal analyst and author of the report, “Worldwide Social Network Ad Revenues: A $10 Billion Market by 2013.” “Social networks have cemented their place in advertising, not just marketing.”

Facebook attracts most of the social marketing spend. U.S. marketers will spend $2.01 billion on Facebook ads this year, up 66.1% from $1.21 billion a year ago. By 2013, that spending will grow another 67.2% to $3.36 billion.

“With $7 of every $10 in social network advertising flowing to the company, Facebook is taking not only a greater share than ever of social network ad spending, but also an increasing proportion of total online ad spending,” says Williamson. “Although its ad offerings and metrics aren’t perfect, marketers still feel they need to be there to reach their target audience.”

While Facebook dominates social marketing, other social networks have recently increased their push for ad dollars. For instance, Twitter last month expanded the reach of its Promoted Tweets paid ad service to enable marketers to reach consumers who aren’t searching for, or following, a retailer’s brand.