Social network sites may be changing people's brains as well as their social life, research suggests.
Brain scans show a direct link between the number of Facebook friends a person has and the size of certain parts of their brain.
It's not clear whether using social networks boosts grey matter or if those with certain brain structures are good at making friends, say researchers.
The regions involved have roles in social interaction, memory and autism.
The work, published in the journal Proceedings of the Royal Society B Biological Sciences, looked at 3-D brain scans of 125 university students from London.
Researchers counted the number of Facebook friends each volunteer had, as well as assessing the size of their network of real friends.
A strong link was found between the number of Facebook friends a person had and the amount of grey matter in certain parts of their brain.
The study also showed that the number of Facebook friends a person was in touch with was reflected in the number of "real-world" friends.
"We have found some interesting brain regions that seem to link to the number of friends we have - both 'real' and 'virtual'," said Dr Ryota Kanai, one of the researchers from University College London.
"The exciting question now is whether these structures change over time. This will help us answer the question of whether the internet is changing our brains."
One region involved is the amygdala, which is associated with memory and emotional responses.
Previous research has shown a link between the volume of grey matter in the amygdala and the size and complexity of real world social networks. Grey matter is the brain tissue where mental processing takes place.
Three other areas of the brain were linked with the size of someone's online social network but not their tally of real-world friends.
The right superior temporal sulcus has a role in perception and may be impaired in autism. The left middle temporal gyrus is associated with "reading" social cues, while the third - the right entorhinal complex - is thought to be important in memory and navigation.
Professor Geraint Rees, from UCL, who led the research, said little is understood about the impact of social networks on the brain, which has led to speculation the internet is somehow bad for us.
"Our study will help us begin to understand how our interactions with the world are mediated through social networks," he said.
"This should allow us to start asking intelligent questions about the relationship between the internet and the brain - scientific questions, not political ones."
Cause and effect
Facebook, the world's most popular social networking site, has more than 800 million active users around the world. The site allows people to keep in touch with friends, from a handful to a thousand or more.
Dr John Williams, Head of Neuroscience and Mental Health at the Wellcome Trust, which funded the study, said: "We cannot escape the ubiquity of the internet and its impact on our lives, yet we understand little of its impact on the brain, which we know is plastic and can change over time.
"This new study illustrates how well-designed investigations can help us begin to understand whether or not our brains are evolving as they adapt to the challenges posed by social media."
Although the study found a link between human brain structure and online social network size, it did not test cause and effect.
Dr Heidi Johansen-Berg, reader in Clinical Neurology at the University of Oxford's Centre for Functional MRI of the Brain, said the study found only a weak relationship between the number of Facebook friends and the number of friends in the real world.
"Perhaps the number of Facebook friends you have is more strongly related to how much time you spend on the internet, how old you are, or what mobile phone you have," she said.
"The study cannot tell us whether using the internet is good or bad for our brains."