The 3% fall is a significant turnaround from previous years, when spending grew an estimated 33% in 2008 and 129% in 2007.
MySpace generated $585m in advertising in 2008, accounting for nearly one-half of US social network ad spend.
EMarketer estimated in December that marketers would spend $630m on in 2009 but the analyst has now reduced its prediction to $495m this year, a 15% drop on 2008.
Outside the US, marketers are expected to spend $25m to advertise on MySpace in 2009.
Debra Aho Williamson, senior analyst at eMarketer, said: "MySpace is still the largest single social networking site when it comes to ad spending, but its fortunes have changed dramatically over the past few months."
News Corp executives said in a May 6 conference call with financial analysts that ad revenues fell 16% year on year at Fox Interactive Media (FIM) in the January to March 2009 quarter. MySpace makes up the bulk of FIM's revenues.
and was replaced by Owen Van Natta, a former Facebook executive.
The decline in ad revenue at MySpace is not anticipated at , which is expected to increase its US ad revenues by 9% in 2009 to $230m.
International spending on Facebook is also expected to jump by $30m this year to hit $70m, for a worldwide total of $300m.
US ad spend on widgets and applications is also projected to rise by 75% from 2008 to reach $70m. US spending on all other social network sites is expected to rise 1% to $345m.
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