  While there is ample evidence that social and communication networks play a
key role during the spread of new ideas, products, or services, network effects
are expected to have diminished influence in the stationary state, when all
users are aware of the innovation, and its usage pattern is determined mainly
by its utility to the user. Here we study four mobile phone-based services
available to over six million subscribers, allowing us to simultaneously
monitor the communication network between individuals and the time-resolved
service usage patterns. We find that usage highly correlates with the structure
of the communication network, and demonstrate the coexistence on the same
social network of two distinct usage classes, network effects being responsible
for the quantifiable differences between them. To test the predictive power of
our theory, we demonstrate that traditional marketing techniques are
ineffective in permanently boosting service adoption, and propose a hub-based
incentive mechanism that has the potential to enhance usage for one of the two
service classes.
