Online song swappers have found plenty of alternatives to Napster
New York-based Jupiter Media Metrix also said the number of Napster users had slumped 31% since the popular website was forced to stop redistributing copyright material in February.
Eight million ex-Napster users are now surfing the net for an alternative means of downloading online music.
And a clutch of file-sharing Napster rivals has sprung up to fight for market share, including Bodetella, Audiogalaxy and Imesh.
Analysts say the fragmentation of the internet song-swapping industry is making it increasingly difficult to draw up laws to protect copyright material from being distributed through these online services.
Fragmentation
"The grim reality is that Napster's audience is beginning to be fragmented across many services, which will be very difficult, if not impossible, to litigate against," said Mark Mooradian, senior analyst at Jupiter Media Matrix, the internet measurement company.
The world's five leading music companies brought the court case against Napster, but may not be prepared to go through the lengthy court battle all over again with a host of rival services.
Similarly, certain record labels have agreed copyright terms with Napster, but may not be prepared to negotiate with a clutch of other song swapping services.
The major record companies themselves have forged plans to launch their own online song services.
But with online music fans already developing preferences for the existing sites, an end to the fragmentation looks increasingly unlikely.
"The next generation of underground file sharing services underscores the need for legitimate music providers, and not just the major labels, to act promptly, putting battles over platforms and copyrights behind them as soon as possible," said Mr Mooradian.
Shrinking Napster
Online Napster usage dropped to 2.2 billion minutes in June 2001, almost a third of its peak usage of 6.3 billion minutes in February.
Amongst Napster's rivals, Audiogalaxy has grown at the fastest rate, with one million American users in May 2001 compared with just 550,000 in March.
Bodetella has taken the number two spot after Napster, with more than a million users in May, while Imesh is also gaining popularity fast.
The figures compiled by Jupiter span 14 countries that together make up 85% of the world's online population.