A report in a science journal has claimed that logging on these sites frequently could increase the risk of cancer and other major illness such as strokes, heart disease and dementia.

"Emailing people rather than meeting up with them may have wide-ranging biological effects," said psychologist Aric Sigman in his report published in Biologist, the journal of the Institute of Biology.

"Increased isolation could alter the way genes work and upset immune responses, hormone levels and the function of arteries. It could also impair mental performance, which could increase the risk of problems as serious as cancer, strokes, heart disease and dementia," the doctor was quoted as saying in his report by Daily Mail.

Even though these sites are designed to bring people together, Sigman said they are making people more isolated. Research suggested that the number of hours people spent speaking to others face-to-face had fallen dramatically since 1987 as the use of electronic media increased.

"Interacting 'in person' had effects on the body not seen when writing emails. Levels of hormones such as the 'cuddle chemical' oxytocin, which promotes bonding, altered according to whether people were in close contact or not," Sigman claimed.

"There does seem to be a difference between 'real presence' and the virtual variety," the doctor added.

Some genes, including ones involved with the immune system and responses to stress, acted differently according to how much social interaction a person had with others, the news daily reported.

"Social networking sites should allow us to embellish our social lives, but what we find is very different. It's not that I'm old fashioned in terms of new technology, but its purpose should be to provide a tool that enhances our lives," Sigman said.

Sigman said 209 'socially regulated' genes had been identified, including ones involved in the immune system, cell proliferation and responses to stress. Their activity may account for higher rates of inflammatory disease and other health problems seen in socially isolated individuals.