  Although recent research on social networks emphasizes microscopic dynamics
such as retweets and social connectivity of an individual user, we focus on
macroscopic growth dynamics of social network link formation. Rather than
focusing on one particular dataset, we find invariant behavior in regional
social networks that are geographically concentrated. Empirical findings
suggest that the startup phase of a regional network can be modeled by a
self-exciting point process. After the startup phase ends, the growth of the
links can be modeled by a non-homogeneous Poisson process with constant rate
across the day but varying rates from day to day, plus a nightly inactive
period when local users are expected to be asleep. Conclusions are drawn based
on analyzing four different datasets, three of which are regional and a
non-regional one is included for contrast.
