Anthony Greco, 18, was lured from New York to Los Angeles under the pretence of a business meeting. He was arrested upon arrival at Los Angeles International Airport last Wednesday.
Greco allegedly sent 1.5 million messages advertising pornography and mortgages. According to reports, the recipients of the messages were all members of the MySpace.com online networking service.
In a further twist upon the scam, Greco had allegedly threatened to share his methods for spamming members of the group if MySpace.com didn't sign an exclusive marketing deal that would have legitimized the messages he was sending via the service.
Greco believed he was flying to Los Angeles to cement that agreement with MySpace President Tom Anderson.
MySpace.com launched IM for its members in December 2003. Reports claim Greco began spamming members using the service in autumn 2004.
Assistant U.S. Attorney Brian Hoffstadt confirmed the arrest was the first criminal case brought against an individual sending spam over IM. But he warned there may well be more to come.
"We're just beginning to get the tip of the iceberg. This could be a new wave as online communities start up," he said in a statement.
Experts have warned in recent years that spim will be the next area of development for those looking to further exploit access to users' desktops and bombard them with unwanted messages, but its growth has been slow.
The immediacy of instant messaging and its growing popularity with businesses and home users means spam via the medium could theoretically become even more obtrusive than email spam.
Will Sturgeon of Silicon.com reported from London.