Experts are concerned children's online social interactions can 'rewire' the brain Computer games and fast-paced TV shows were also a factor, she said.
Lady Greenfield told the Lords a teacher of 30 years had told her she had noticed a sharp decline in the ability of her pupils to understand others.
'It is hard to see how living this way on a daily basis will not result in brains, or rather minds, different from those of previous generations,' she said.
She pointed out that autistic people, who usually find it hard to communicate, were particularly comfortable using computers.
Social networking websites are causing alarming changes in the brains of young users, an eminent scientist has warned.
Sites such as Facebook, Twitter and Bebo are said to shorten attention spans, encourage instant gratification and make young people more self-centred.
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. The popular website has made him a very rich man, but at what cost to human relationships?
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.