  With the advancement in the information age, people are using electronic
media more frequently for communications, and social relationships are also
increasingly resorting to online channels. While extensive studies on
traditional social networks have been carried out, little has been done on
online social network. Here we analyze the structure and evolution of online
social relationships by examining the temporal records of a bulletin board
system (BBS) in a university. The BBS dataset comprises of 1,908 boards, in
which a total of 7,446 students participate. An edge is assigned to each
dialogue between two students, and it is defined as the appearance of the name
of a student in the from- and to-field in each message. This yields a weighted
network between the communicating students with an unambiguous group
association of individuals. In contrast to a typical community network, where
intracommunities (intercommunities) are strongly (weakly) tied, the BBS network
contains hub members who participate in many boards simultaneously but are
strongly tied, that is, they have a large degree and betweenness centrality and
provide communication channels between communities. On the other hand,
intracommunities are rather homogeneously and weakly connected. Such a
structure, which has never been empirically characterized in the past, might
provide a new perspective on social opinion formation in this digital era.
