Collective action taking place on Internet platforms leaves a digital imprint which may be harvested to better understand the dynamics of mobilization. This ‘big data’ offers social science researchers the potential for new forms of analysis, using real-time transactional data based on entire populations, rather than sample-based surveys of what people think they did or might do. This paper uses a big data approach to track the growth of about 20,000 petitions to the UK Government over two years, analyzing the rate of growth and the outreach mechanism.