Online social networks (OSNs) enable researchers to study the social universe
at a previously unattainable scale. The worldwide impact and the necessity to
sustain their rapid growth emphasize the importance to unravel the laws
governing their evolution. We present a quantitative two-parameter model which
reproduces the entire topological evolution of a quasi-isolated OSN with
unprecedented precision from the birth of the network. This allows us to
precisely gauge the fundamental macroscopic and microscopic mechanisms
involved. Our findings suggest that the coupling between the real pre-existing
underlying social structure, a viral spreading mechanism, and mass media
influence govern the evolution of OSNs. The empirical validation of our model,
on a macroscopic scale, reveals that virality is four to five times stronger
than mass media influence and, on a microscopic scale, individuals have a
higher subscription probability if invited by weaker social contacts, in
agreement with the "strength of weak ties" paradigm