FACEBOOK may be changing people's brains as well as their social world.
Scientists have found a direct link between the number of "Facebook friends" a person has and the size of specific parts of the brain.
The regions involved have roles in social interaction as well as memory and at least one is implicated in autism.
It could be that the differences seen are due to the effects of online activity on the brain.
Alternatively, people with certain brain traits may be more likely to have larger numbers of friends, both on Facebook and in the real world.
"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 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."
Facebook, the world's most popular social networking site, has more than 800 million active users across the globe.
While some users have only a handful of online friends, others list more than 1,000.
Professor Geraint Rees, who led the research, said: "Online social networks are massively influential, yet we understand very little about the impact they have on our brains.
"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."
The scientists studied brain scans of 165 university students, all active Facebook users, and assessed the size of their real and online friendship networks.
A strong correlation was seen between numbers of Facebook friends and the amount of "grey matter" in regions of the brain.
One of the highlighted regions was the amygdala, a bundle of nerves associated with memory, emotional responses and empathy.
Prof Rees said: "Our findings support the idea that most Facebook users use the site to support their existing social relationships, rather than just creating networks of entirely new, virtual friends."