  How are people linked in a highly connected society? Since in many networks a
power-law (scale-free) node-degree distribution can be observed, power-law
might be seen as a universal characteristics of networks. But this study of
communication in the Flickr social online network reveals that power-law
node-degree distributions are restricted to only sparsely connected networks.
More densely connected networks, by contrast, show an increasing divergence
from power-law. This work shows that this observation is consistent with the
classic idea from social sciences that similarity is the driving factor behind
communication in social networks. The strong relation between communication
strength and node similarity could be confirmed by analyzing the Flickr
network. It also is shown that node similarity as a network formation model can
reproduce the characteristics of different network densities and hence can be
used as a model for describing the topological transition from weakly to
strongly connected societies.
