Let G be a graph and τ be an assignment of nonnegative thresholds to the vertices of G. A subset of vertices, D, is an irreversible dynamic monopoly of (G, τ) if the vertices of G can be partitioned into subsets D 0 , D 1 , . . . , D k such that D 0 = D and, for all i with 0 ≤ i ≤ k − 1, each vertex v in D i+1 has at least τ(v) neighbours in the union of D 0 , D 1 , . . . , D i . Dynamic monopolies model the spread of influence or propagation of opinion in social networks, where the graph G represents the underlying network. The smallest cardinality of any dynamic monopoly of (G, τ) is denoted by dyn τ (G). In this paper we assume that the threshold of each vertex v of the network is a random variable X v such that 0 ≤ X v ≤ deg G (v) + 1. We obtain sharp bounds on the expectation and the concentration of dyn τ (G) around its mean value. We also obtain some lower bounds for the size of dynamic monopolies in terms of the order of graph and expectation of the thresholds.2010 Mathematics subject classification: primary 05C69; secondary 91D30.