Social networking websites such as Facebook and MySpace aren't expanding people's circles of close friends, but they are creating plenty of meaningless relationships, according to British researchers.

A study of the sites revealed that while many users have hundreds or even thousands of acquaintances on their accounts, their core group of close friends is still unchanged at around five people. However, weak ties among people around the globe are rising exponentially, said Will Reader, an evolutionary psychologist at Sheffield Hallam University, at a meeting of the British Association for the Advancement of Science.

The online study used a questionnaire and, based on the first 200 responses, found that close friendships were formed through in-person meetings in an overwhelming 90 per cent of cases.

"Face-to-face contact is a requirement for intimate friendships," he told the conference. "There are many emotional cues that people give face to face, such as smiling and laughing, which are impossible to fake, whereas online it is easy to say, 'You are wonderful, I love you.' "

A previous study done at the University of Liverpool found that most people have an average of 150 acquaintances in their social network yet also maintain a small core group of friends, which may indicate limitations on the human brain.

Reader said there are "good evolutionary reasons" why core friendship groups are so small. Making friendships means investing time and even money in another person, in which case face-to-face contact is invaluable so that people can see whether their investment is worthwhile. On the internet, it is "very easy to be deceptive" he said.

Social networking sites are, however, giving rise to a group of people known as friend collectors, who add little-known acquaintances just for the sake of having a large number of contacts on their profile. This allows people to "collect friends like boys collect Airfix models."