'Dr Aric Sigman says the amount of time we spend with each other has slumped dramatically and in turn is damaging our health.
He says our devotion to such sites could alter the way genes work, upset immune responses, hormone levels, and the function of arteries, and influence mental performance.
Levels of hormones such as the "cuddle chemical" oxytocin, which promotes bonding, altered according to whether people were in close contact or not.
This could increase the risk of health problems as serious as cancer, strokes, heart disease, and dementia.
Dr Sigman spells out his warning in the latest issue of Biologist, the journal of the Institute of Biology, and maintains that social networking sites have played a significant role in people becoming more isolated.
He said: "Social networking is the internet's biggest growth area, particular among young children.
"A quarter of British children have a laptop or computer in their room by the age of five and they have their own social networking sites, like the BBC's myCBBC. It's causing huge changes."
Dr Sigman said 209 "socially regulated" genes have been identified, including ones involved in the immune system, cell proliferation and responses to stress.
Electronic media is also undermining the ability of children and young people to learn vital social skills and read body language, he said.
Dr Sigman continued: "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.
"In less than two decades, the number of people saying there is no one with whom they discuss important matters nearly tripled.
"Parents spend less time with their children than they did only a decade ago. Britain has the lowest proportion of children in all of Europe who eat with their parents at the table. The proportion of people who work at home alone continues to rise.
"I am worried about where this is all leading. 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.
"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."
Research suggests the number of hours people spend interacting face-to-face has fallen dramatically since 1987 as electronic media use increases.
From: Yahoo News, ITN
