  The role of committed minorities in shaping public opinion has been recently
addressed with the help of multi-agent models. However, previous studies
focused on homogeneous populations where zealots stand out only for their
stubbornness. Here, we consider the more general case in which individuals are
characterized by different propensities to communicate. In particular, we
correlate commitment with a higher tendency to push an opinion, acknowledging
the fact that individuals with unwavering dedication to a cause are also more
active in their attempts to promote their message. We show that these
\textit{activists} are not only more efficient in spreading their message but
that their efforts require an order of magnitude fewer individuals than a
randomly selected committed minority to bring the population over to a new
consensus. Finally, we address the role of communities, showing that partisan
divisions in the society can make it harder for committed individuals to flip
the status-quo social consensus.
