Sites such as Facebook, Twitter and Bebo are said to shorten attention spans, encourage instant gratification and make young people more self-centred.
Experts are concerned children's online social interactions can 'rewire' the brain
'We are seeing children's brain development damaged because they don't engage in the activity they have engaged in for millennia.
Social networking websites are causing alarming changes in the brains of young users, an eminent scientist has warned.
'I'm not against technology and computers. But before they start social networking, they need to learn to make real relationships with people.'
The claims from neuroscientist Susan Greenfield will make disturbing reading for the millions whose social lives depend on logging on to their favourite websites each day.
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But they will strike a chord with parents and teachers who complain that many youngsters lack the ability to communicate or concentrate away from their screens.
More than 150million use Facebook to keep in touch with friends, share photographs and videos and post regular updates of their movements and thoughts.
A further six million have signed up to Twitter, the 'micro-blogging' service that lets users circulate text messages about themselves.
But while the sites are popular - and extremely profitable - a growing number of psychologists and neuroscientists believe they may be doing more harm than good.
Baroness Greenfield, an Oxford University neuroscientist and director of the Royal Institution, believes repeated exposure could effectively 'rewire' the brain.