  Diffusion of information, behavioral patterns or innovations follows diverse
pathways depending on a number of conditions, including the structure of the
underlying social network, the sensitivity to peer pressure and the influence
of media. Here we study analytically and by simulations a general model that
incorporates threshold mechanism capturing sensitivity to peer pressure, the
effect of `immune' nodes who never adopt, and a perpetual flow of external
information. While any constant, non-zero rate of dynamically-introduced
spontaneous adopters leads to global spreading, the kinetics by which the
asymptotic state is approached shows rich behavior. In particular we find that,
as a function of the immune node density, there is a transition from fast to
slow spreading governed by entirely different mechanisms. This transition
happens below the percolation threshold of network fragmentation, and has its
origin in the competition between cascading behavior induced by adopters and
blocking due to immune nodes. This change is accompanied by a percolation
transition of the induced clusters.
