Analysis of online networks can identify cascades as they flow through communities but, often, the reasons for their initiation are tacit. This paper draws on social capital theory to investigate the characteristics and behaviours that can lead to this phenomenon. The relationships between structural, relational and cognitive sources of social capital and two dimensions of influence are investigated using 1,970 respondents from three large-scale online communities. The model of influence that leads to the intention to diffuse a message is strikingly similar to that of direct influence; presenting strong support to the notion that contagion-based cascades through networks are predictors of persuasion. The paper proposes a bridge between the theories of social capital and personal influence, which have not previously been discussed. Further, techniques are suggested which can help organisations to identify opinion-leaders and, if required, subvert or redirect the nature of their influence.