Business-to-business (B2B) marketers are starting to advertise more heavily on social network sites. eMarketer projects that U.S. marketers will spend $40 million this year advertising to a business audience on social network sites. As the number of business users of social networks increases, advertising expenditures will rise accordingly, reaching an estimated $210 million in 2012. The market has seen a sizable bump in the past year, growing an estimated 169% between 2007 and 2008. Spending is expected to double next year, to $80 million. B2B advertising on social networks remains a very small portion of the overall U.S. online social network ad market.