An interesting study was published last month in the journal Proceedings of the Royal Society about a relationship between the number of friends in people's online social networks, such as Facebook, and the density of their brain matter in regions implicated in social cognitive function.
In their study the authors collected the structural MRI scans from a sample of 125 healthy adults, as well as a replication sample of 40 volunteers. Using this data, the authors assessed "whether variability in the structure of specific regions of human cortex was associated with inter-individual variability in the number of social relationships as indexed by Facebook."
In discussing their findings, the authors concluded that variations in the number of Facebook friends were able to "strongly and specifically" predict grey matter volume in the left MTG, right STS, and right entorhinal cortex. They were also able to support a previous finding that the real-world social network size, which was found to correlate with the grey matter density of the amygdala (responsible for memory and emotional response), also exhibits this correlation with online social network size.
In concluding, the authors state that their results "now suggest that the posterior STS and MTG, implicated in social perception, and the entorhinal cortex, implicated in associative memory, provide the cognitive capacity to build and maintain large online social networks in human society. Moreover, they show a link between variability in structural (rather than functional) properties of the human brain and online social network size."
Limitations of the study include the fact the the study population was exclusively comprised of college students, mainly because of their relatively high usage of online social networks. However, this limits the ability to ascertain whether the correlations of the study hold true for different demographic groups. The authors also noted that the cross-sectional nature of the study limits the ability to determine the nature of the correlation -- is it that students who are more extroverted have a greater ability to make larger networks, or is it larger social networks that continue to shape and influence brain structure?