A slew of upstart rivals and the "chronic unfaithfulness" of internet users have taken a bite out of MySpace’s audience and could see the site fall from the top spot in the social network rankings by September.

The number of British visitors to MySpace dipped to 6.5 million in May, from 6.8 million in April, according to Nielsen//Netratings, the market researchers.

The fall, which comes as the nearest rival, Facebook, sees a huge surge in traffic, is the first to hit MySpace since it signed a $900 million advertising deal with Google last summer and only the second since News Corporation bought the group for $580 million two years ago.

The pattern was repeated in the United States, MySpace's largest market, where traffic to the site fell to 56.6 million in May, from 57 million a month earlier.

News Corp is the parent company of Times Online

The situation has already caught the attention of Rupert Murdoch, the chief executive of News Corp.

Asked in a recent interview whether newspaper readers are migrating to MySpace, he said: "I wish they were. They're all going to Facebook at the moment."

MySpace has so far hit with ease the traffic targets set down in last year's deal with Google. But the dip in its users underscores the problem of "networking promiscuity", which sees website owners battle to keep flighty users engaged in a market where the majority of services are free.

A recent survey of social networking sites by Parks Associates, the analysts, found that social networkers are "chronically unfaithful" with nearly half regularly using more than one site and one in six using three or more.

The Nielsen's figures confirm that trend, showing that 444,000 Britons visited all three of MySpace, Bebo and Facebook in May.

The numbers also pose questions over the sustainability of web-based brands, which can soar to global prominence in a matter of months.

Rita Clifton, the chairman of Interbrand, the consultancy, said: "The internet has allowed a brand-building process that would have once taken decades to be achieved in a fraction of that. There is a downside, of course: what goes up quickly can descend just as fast."

Ms Clifton added that there are suggestions that web users are "graduating" from older sites, such as MySpace, to newer sites such as Facebook, which are perceived to be more "grown-up".

According to Nielsen, over the past six months, Facebook's audience in the UK has grown at 19 times the rate of MySpace's, surging 523 per cent.

Alex Burmaster, of Nielsen, said: "MySpace is, by far, still the most popular social network. However, if last month's growth rates were to remain consistent … Facebook would catch MySpace in September."

Park Associates suggested that the wandering behaviour of social network users could open opportunities for new sites and software developers who build applications that can link different networks.

John Barrett, who led the group’s research, said: “MySpace is a growing ecosystem and one that ironically now extends beyond MySpace itself.”

MySpace sought to reinvigorate its user base yesterday when it announced the launch of a new video-sharing service, MySpaceTV, which will compete with YouTube, the rival site owned by Google.

Last month, to the same end, Facebook launched a new "platform" that allows outside developers to create online services, such as photo slideshows and online data storage, which can be operated from inside its site.

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