Users of social networking sites such as Facebook and MySpace are “chronically unfaithful” with nearly half regularly using more than one site and one in six using three or more, according to a report.

The findings, by Parks Associates, the analysts, underscore the problems faced by social networks in keeping young flighty users engaged in a market where the majority of services are free to join and use.

It is a conundrum to have already caught the attention of Rupert Murdoch, the chief executive of News Corporation, which owns My Space, the largest social network site.

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."

News Corp is also the parent company of Times Online.

The infidelity of users also raises questions over the long-term futures of the business models embraced by social networks, who rely heavily on their "stickiness" — or ability to keep audiences on their sites for long periods — to sell advertising space at premium rates.

According to Park Associates nearly 40 per cent per cent of users of MySpace also keep profiles on rival sites such as Friendster and Facebook.

Loyalty among the smaller social networking sites was lower, with more than 50 per cent of all users actively maintaining multiple profiles across different sites.

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 research, said: “MySpace is a growing ecosystem and one that ironically now extends beyond MySpace itself.”

According to comScore, the web analysts, 105 million people visited MySpace in April while 38.8 million visited Facebook.

Last month, 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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