In 2011, a vast coalition of social movement actors coordinated one of the largest and most successful political campaign that characterized recent Italian history, organizing and winning a referendum that blocked a serious attempt to privatize the entire water distribution network. In a year characterized by intense mobilizations throughout the world – with the Occupy, the 15-M and the so called Arab Spring protests dominating the scene – the main Italian organizations and networks coalesced, with the external support of some small declining or newly formed parties, and gradually captured an increasing attention in society. The main environment of action of the Referendum supporters slowly passed from the Italian streets, assemblies, and squares, to the websites of the organizations, and – during the very last phase of the campaign – to Facebook, finally conquering at least in part the very closed space of the Italian mass media. On Facebook, in particular, the politically oriented communication of the referendum supporters proved to be very pervasive: the words referendum and quorum were the most present in the statuses of the Italian users of this platform for the entire 2011. The dissertation explores in detail this successful campaign, focusing on how the activists elaborated new strategies of online communication and on the processes of adaptation that the emergence of the Social Media in the Italian political environment promoted in this social movement milieu. Adopting a very wide set of methodologies, which includes Digital Ethnography, Social Network Analysis, interviews and data collection through computer programming in Python, the author explores different aspects of the mobilization that are particularly relevant for the broader discussion on online activism and campaigning. In particular, he traces the network of websites that supported the campaign, he observes the online communication practices of the activists on the web and Facebook, he describes the link that connects online and off-line activism during this large-scale campaign, and he connects the different ways of perceiving the social media environment with divergent uses of these platforms.