Social Media are the privileged channel for news spreading and checking. Unexpectedly for many users, automated accounts, known as social bots, contribute more and more to this process of information diffusion. In the present talk we present how different skills in Computer Science and Network theory provide complementary information for the comprehension of the role of social bots in the social network propaganda. Using Twitter as a benchmark, we consider the traffic exchanged, over one month of observation, on the migration flux from Northern Africa to Italy. We measure the significant traffic of tweets only, by implementing an entropy-based null model that discounts the activity of users and the virality of tweets. Results show that social bots play a central role in the exchange of significant content. Indeed, not only the strongest hubs have a number of bots among their followers higher than expected, but furthermore a group of them, that can be assigned to the same political tendency, share a common set of bots as followers. The retweeting activity of such automated accounts amplifies the hubs' messages.