The research was conducted by researchers form Aalto University and University of Oxford, and was published in Proceedings of the National Academy of Sciences of the United States of America (PNAS) recently.
"We found that study participants focus the majority of their communications on a small number of network members. In addition, there was distinctive, individual variation in the exact way study participants allocate their limited communication time across network members. These distinct patterns persist and retain their characteristic shape over time, even when members of a person's network change. These findings may reflect limitations in the human ability to maintain many emotionally close relationships", tells Jari Saramäki.
The research combined survey data and detailed data from mobile phone call records that were used to track changes in the communication networks of 24 students in the UK over 18 months as they made the transition from school to university or work. The call records provided a complete list of time-stamped calls made by study participants to everyone in their networks. The surveys provided specific information about the people in a network including emotional closeness, time between face-to-face contact and all phone numbers attached to specific network members.
Link to the PNAS-article: "The persistence of social signatures in human communication"