Online social networking advertising spending worldwide reached US$2 billion in 2008, and is expected to grow to $3.49 billion in 2013, according to eMarketer.

The spending increased with a growth of 46 percent in 2008. In the following years, although the rise is predicted to slow, it is still expected to maintain a double digit growth rate until 2013, when it is forecast to see a 9.6 percent annual increase, SFN's World Digital Media Trends 2009 reported.

In 2008, MySpace accounted for the majority of the overall online social networking ad spending, with 54 percent, or $850 million. Facebook came next with 20 percent, or $305 million. Other general or niche social networking sites made up the rest.

In Western Europe, online social networking ad spending reached $170 million in 2007, and is expected to rise to $970 million in 2012, according to eMarketer. The United Kingdom is the key market in the region, contributing $130 million in 2007, and expected to increase to $533 million in 2012.

The United Kingdom accounted for more than 75 percent of the overall Western Europe market in 2007, but eMarketer projects its share will continue to fall, and in 2012 will contribute about 55 percent, according to the report, World Digital Media Trends 2009, released by SFN and the World Association of Newspapers and News Publishers.