In this paper we consider chance curation (the task of easing chance-discovery activities for users) as far as it concerns information sharing in online communities, understood as virtual cognitive niches. Virtual cognitive niches can be considered as digitally-encoded collaborative distributions of information and pieces of knowledge into the environment. The scope of chance curation within these contexts depends on the quality of the information externalized, the aptness of the information-sharing mechanisms to the agents' purposes and the control they can implement over the system. Online communities, as socially biased networks, provide more ways to connect the users to each other than to control the quality of the information they share and receive. We contend that this social bias shapes chance curation into the discrimination between what Nagy and Neff called ``imagined affordances'' (the combination of users' perceptions, attitudes and expectations over the functionality of a particular technology) and what we can call ``critical chance'', which are event-related data that conceal a particularly good opportunity or a particularly dreadful risk for the agent and her surroundings.