Social network advertising clearly has a bright future, and the prospects for future growth were confirmed by a new forecast from Borrell Associates, which predicts massive growth in ad revenues this year. My only question is: Where is this ad spending going to happen? Overall the Borrell report, "The Social Networking Explosion: Ad Revenue Outlook" has social network ad spending increasing 68% from $4 billion in 2009 to $7.5 billion in 2010, then continuing to grow every year to about $38 billion in 2015. The 2015 figure will represent approximately a third of all U.S. online marketing spending. According to Borrell, 2009 ad spending was divided roughly equally between national brands, while half came from local advertisers. Meanwhile, approximately a quarter of the spending actually goes to promotions, not advertising. For all these caveats, however, I'm still at a loss to figure out where the majority of the dollars are coming from, and where they are going to; I'm also not sure how to make the Borrell figures line up with other (much lower) estimates of social network ad spending. For example, a report from eMarketer released in December 2009 estimated social network ad revenues in 2009 at $2.2 billion, a little over half the Borrell estimate. Even factoring in another third for the additional dollars which Borrell says go to promotions, that still comes to just $2.9 billion -- meaning there's over $1 billion missing. While that might be a rounding error for, say, the U.S. budget, it's a lot compared to the relatively small spending base of this young, growing medium. Moreover, I have to wonder which social networks account for the current spending, as well as for the projected future growth. According to Facebook CEO Mark Zuckerberg, Facebook might make $1 billion in 2010, up from about $800 million in 2009. The separate (December 2009) figures from eMarketer have MySpace revenues at $465 million in 2009, decreasing 23% to $360 million in 2010. Twitter execs forecast $140 million in ad revenue in 2010, while Classmates.com had about $80 million in ad revenue in 2009 and is on course for similar performance in 2010. All that adds up to about $1.35 billion in 2009 and $1.58 billion in 2010; depending on which estimate you are using -- Borrell or eMarketer -- that means there was somewhere between $850 million and $2.65 billion which went to other networks in 2009, and almost $6 billion going to other networks in 2010 (per Borrell). Understanding the peril of combining figures from different sources, and acknowledging that there are probably a number of smaller networks adding incremental ad revenues, I'm still at a loss to make the estimates even come close to adding up.