There's a direct link between the number of Facebook friends a person has and the size of particular brain regions, say researchers at University College London (UCL).
And the more Facebook friends a person has, the more real-world friends they are likely to have too.
"Online social networks are massively influential, yet we understand very little about the impact they have on our brains. This has led to a lot of unsupported speculation the internet is somehow bad for us," says Professor Geraint Rees, a Wellcome Trust Senior Clinical Research Fellow at UCL.
"Our study will help us begin to understand how our interactions with the world are mediated through social networks. This should allow us to start asking intelligent questions about the relationship between the internet and the brain – scientific questions, not political ones."
Rees's team studied brain scans of 125 university students – all active Facebook users – and compared them against the size of the students' network of friends, both online and in the real world.
They found a strong connection between the number of Facebook friends an individual had and the amount of grey matter in several regions of the brain, including the amygdala - a region associated with processing memory and emotional responses.
A recent study showed that there's more grey matter in this area in people with lots of real world friends – and it now seems the same is true for online friends.
The size of three other regions – the right superior temporal sulcus, the left middle temporal gyrus and the right entorhinal cortex – also correlated with online social networks, but did not appear to correlate with real-world networks.
The superior temporal sulcus helps us perceive a moving object as biological, and structural defects in this region have been identified in some children with autism. The entorhinal cortex, meanwhile, has been linked to memory and navigation – including navigating through online social networks.
Finally, the middle temporal gyrus has been shown to activate in response to the gaze of others, and so is implicated in perception of social cues.
The researchers stress that they've found a correlation and not a cause: in other words, it's not clear whether having more Facebook friends makes these regions of the brain larger or whether some people are 'hard-wired' to have more friends.
"We have found some interesting brain regions that seem to link to the number of friends we have – both 'real' and 'virtual'," says Dr Ryota Kanai, first author of the study.
"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."
As well as brain structure, the researchers also examined whether there was a link between the sizes of a person's online and real world network network of friends.
They asked their volunteers questions such as how many people they'd text about a celebratory event such as a birthday or new job, how many friends were in their phonebook, and how many friends they've kept from school and university.
"Our findings support the idea that most Facebook users use the site to support their existing social relationships, maintaining or reinforcing these friendships, rather than just creating networks of entirely new, virtual friends," says Rees.