  Adoption of innovations, products or online services is commonly interpreted
as a spreading process driven to large extent by social influence and
conditioned by the needs and capacities of individuals. To model this process
one usually introduces behavioural threshold mechanisms, which can give rise to
the evolution of global cascades if the system satisfies a set of conditions.
However, these models do not address temporal aspects of the emerging cascades,
which in real systems may evolve through various pathways ranging from slow to
rapid patterns. Here we fill this gap through the analysis and modelling of
product adoption in the world's largest voice over internet service, the social
network of Skype. We provide empirical evidence about the heterogeneous
distribution of fractional behavioural thresholds, which appears to be
independent of the degree of adopting egos. We show that the structure of
real-world adoption clusters is radically different from previous theoretical
expectations, since vulnerable adoptions --induced by a single adopting
neighbour-- appear to be important only locally, while spontaneous adopters
arriving at a constant rate and the involvement of unconcerned individuals
govern the global emergence of social spreading.
