People's health could be harmed by social networking sites because they reduce levels of face-to-face contact, an expert claims.
Dr Aric Sigman says websites such as Facebook set out to enrich social lives, but end up keeping people apart.
Dr Sigman makes his warning in Biologist, the journal of the Institute of Biology.
A lack of "real" social networking, involving personal interaction, may have biological effects, he suggests.
He also says that evidence suggests that a lack of face-to-face networking could alter the way genes work, upset immune responses, hormone levels, the function of arteries, and influence mental performance.
This, he claims, could increase the risk of health problems as serious as cancer, strokes, heart disease, and dementia.
Dr Sigman maintains that social networking sites have played a significant role in making people become more isolated.
"Social networking is the internet's biggest growth area, particular among young children," he said.
"Social networking sites should allow us to embellish our social lives, but what we find is very different. The tail is wagging the dog. These are not tools that enhance, they are tools that displace."
Dr Sigman says that there is research that suggests the number of hours people spend interacting face-to-face has fallen dramatically since Protected content , as the use of electronic media has increased.
And he claims that interacting "in person" has an effect on the body that is not seen when e-mails are written.
"When we are 'really' with people different things happen," he said.
"It's probably an evolutionary mechanism that recognises the benefits of us being together geographically.
"Much of it isn't understood, but there does seem to be a difference between 'real presence' and the virtual variety."
Dr Sigman also argues using electronic media undermines people's social skills and their ability to read body language.
"One of the most pronounced changes in the daily habits of British citizens is a reduction in the number of minutes per day that they interact with another human being," he said.
"In less than two decades, the number of people saying there is no-one with whom they discuss important matters nearly tripled."
Dr Sigman says he is "worried about where this is all leading".
He added: "It's not that I'm old fashioned in terms of new technology, but the purpose of any new technology should be to provide a tool that enhances our lives."