Apart from mechanisms to make crowd-sourcing secure, the reliability of a collaborative system is dependent on the economic incentives of its potential contributors. We study several factors related to the incentives in a community-based security system, including the expectation on the social influence and the contagion effect of generosity. We also investigate the effects of organizing community members differently in a complete, random and scale-free structure. Our simulation results show that, without considering any specific incentive schemes, it is not easy to encourage user contribution in a complete-graph community structure (global systems). On the other hand, a moderate level of cooperative behavior can be cultivated when the community members are organized in the random or scale-free structure (social networks).