In this paper, we argue for the benefits of enabling the self-organisation of virtual online communities, which today are mainly formed and operated by centrally managed web servers. However, self-organisation requires community members themselves to contribute different types of resources (e.g., bandwidth, storage, etc.), as in the case of Peer-to-Peer (P2P) systems. Unfortunately, this cooperation cannot be taken for granted. To address this obstacle, we introduce the notion of a cross-layer incentive mechanism. The main idea is to encourage the contribution of low-level resources using social incentives generated at a higher (social) layer. We believe that this type of incentive mechanism will play a central role toward the realisation of self-organised virtual communities and enable users to take advantage of the attractiveness and value of web-based communities, on the one hand, and the externalities and flexibility of P2P networks, on the other hand. We make a first step toward this direction: a) we categorise the different types of social incentives applicable in this context and b) we provide insights for the design of the appropriate social software required to map the behaviour of the participants at the resource-sharing layer with suitable rewards at the social layer.