  In this paper we define a metric for reciprocity---the degree of balance in a
social relationship---appropriate for weighted social networks in order to
investigate the distribution of this dyadic feature in a large-scale system
built from trace-logs of over a billion cell-phone communication events across
millions of actors. We find that dyadic relations in this network are
characterized by much larger degrees of imbalance than we would expect if
persons kept only those relationships that exhibited close to full reciprocity.
We point to two structural features of human communication behavior and
relationship formation---the division of contacts into strong and weak ties and
the tendency to form relationships with similar others---that either help or
hinder the ability of persons to obtain communicative balance in their
relationships. We examine the extent to which deviations from reciprocity in
the observed network are partially traceable to these characteristics.
