Most person-to-person communication occurs between persons who know each other well (family and close friends). A recent study of a large number of mobile phone voice calls found that in an 18-week period, about two-thirds of mobile phone users engaged in mutual calling with only two other persons. The mean number of partners for mutual calling in that period was three. Mutual calling partners with more mutual calling partners in common spent on average more time in calls with each other. Mutual calling appears to have increasing returns in personal familiarity.
Recent neuroscience research points to neural functioning that supports this macro-behavioral pattern. The suppression of a certain brain wave pattern (mu activity) is associated with sub-conscious processing of the present activity of another person. Premotor neurons that perform such sub-conscious processing have been called mirror neurons. Mu activity, and by implication mirror neuron activity, depends on familiarity with the other person:
mu activity was suppressed most when subjects watched videos of themselves, indicating the greatest mirror neuron activity. For both groups [autistic and non-autistic children], the measurements showed a slightly lower level of suppression when subjects watched familiar people in the video and the least when watching strangers.
Recognition of another is typically considered to be a high-level neural function. Familiarity with another, however, appears to associated with (downloaded) resources for sub-conscious processing of another’s actions.
Persons highly value in communication making sense of presence. The relation of mu activity to personal familiarity is consistent with personal familiarity being a resource for making sense of presence. Presence as a value, and familiarity as a resource, provide a structure for increasing returns in mutual calling.
 See Analysis of a large-scale weighted network of one-to-one human communication, Jukka-Pekka Onnela et al 2007 New J. Phys. 9 179 doi:10.1088/1367-2630/9/6/179, Fig. 4 and Table 1.
 See Structure and tie strengths in mobile communication networks, J.-P. Onnela, J. Saramäki, J. Hyvönen, G. Szabó, D. Lazer, K. Kaski, J. Kertész, and A.-L. Barabási, PNAS 104, 7332-7336 (2007), preprint, pp. 4-5. Overview of this paper here.
 From “Mirror, Mirror In The Brain: Decoding Patterns Reflecting Understanding Of Self, Others May Further Autism Therapies,” Society for Neuroscience News Release, 11/04/07, summarizing L. M. Oberman, V. S. Ramachandran, J. A. Pineda, “Mirror Neuron Activity Modulated by Actor Familiarity in Children with Autism Spectrum Disorders: an EEG Study,” 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007, abstract on EBDblog. The sentence following the above quoted excerpt states, “This indicates that normal mirror neuron activity was evoked when children with autism watched family members, but not strangers.” The abstract states, “Both neurotypical participants and those with ASD [autism spectrum disorder] showed greater suppression to familiar individuals compared to the stranger.” These and the above descriptions appear to be inconsistent, but that is not relevant to the point here.