  We study an opinion formation model by the means of a co-evolving complex
network where the vertices represent the individuals, characterised by their
evolving opinions, and the edges represent the interactions among them. The
network adapts to the spreading of opinions in two ways: not only connected
agents interact and eventually change their thinking but an agent may also
rewire one of its links to a neighborhood holding the same opinion as his. The
dynamics depends on an external parameter {\Phi}, which controls the plasticity
of the network. We show how the information entropy associated to the
distribution of group sizes, allows to locate the phase transition between full
consensus and a society where different opinions coexist. We also determine the
minimum size of the most informative sampling. At the transition the
distribution of the sizes of groups holding the same opinion is scale free.
