As the number of business users of social networks continues to increase, advertising expenditures will rise, too says eMarketer.  In the US this year, advertisers will spend $40 million to reach a business audience on online social networks and it will reach $210 million in 2012.

“As compelling as the ad growth is, marketers will spend far more over the next few years to create and manage their own social networks for business customers, partners, suppliers and vendors,” says Debra Aho Williamson, senior analyst at eMarketer and author of the new report, B2B Marketing on Social Networks: Engaging the Business Audience. “These business networks will serve a range of purposes, from improving customer communication and collaboration to aiding product development.”

A clear sign of the revolution in business thinking and behavior is the popularity of LinkedIn—the most widely known professional social network—where the audience has more than doubled in the past year, and the rise of additional business-oriented networks, including a raft of networks aimed at verticals.

In fact, vertical industry networks targeting just about any job description or category are proliferating such as ITtoolbox and WebGuild for web and IT industry; Legal OnRamp and LawLink for attorneys; and Reel-Exchange, for film and video professionals.

“Even Facebook has become a de facto B2B social network,” says Ms. Williamson, “simply because so many business executives have joined it in the past year.” In June 2008 the deal with Visa, which created an application called Visa Business Network for small businesses on Facebook, signals that it will probably attract more advertising aimed at a growing business audience.

“With the growing popularity of professional social network Websites, businesspeople are changing how they build professional relationships and find jobs,” says Ms. Williamson.