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."