This paper examines the size of social networks in contemporary Western society, focusing on the exchange of Christmas cards as a means to understand network structure and contact frequency. The study found that the maximum network size averaged 153.5 individuals, with a mean network size of 124.9 for those explicitly contacted. These values align closely with the 150-group size predicted for humans based on neocortex size. Age, household type, and relationship to the individual influence network structure, with the proportion of kin remaining around 21%. Frequency of contact is influenced by passive factors (distance, work colleague, overseas) and active factors (emotional closeness, genetic relatedness). Controlling for passive factors allowed the hierarchical structure of human social groups to be identified, suggesting cognitive constraints on network size. The findings support the idea that social networks in modern societies reflect the same hierarchical structure observed in traditional societies, indicating that cognitive constraints on network size may be universal.This paper examines the size of social networks in contemporary Western society, focusing on the exchange of Christmas cards as a means to understand network structure and contact frequency. The study found that the maximum network size averaged 153.5 individuals, with a mean network size of 124.9 for those explicitly contacted. These values align closely with the 150-group size predicted for humans based on neocortex size. Age, household type, and relationship to the individual influence network structure, with the proportion of kin remaining around 21%. Frequency of contact is influenced by passive factors (distance, work colleague, overseas) and active factors (emotional closeness, genetic relatedness). Controlling for passive factors allowed the hierarchical structure of human social groups to be identified, suggesting cognitive constraints on network size. The findings support the idea that social networks in modern societies reflect the same hierarchical structure observed in traditional societies, indicating that cognitive constraints on network size may be universal.