Currently there is much interest in the use of Web 2.0 technologies to support knowledge sharing in organisations. Many successful projects have been reported. These reports emphasise how the use of such technology has unlocked new pathways for knowledge transfer. However, the limitations of Web 2.0 technologies are not yet well understood and potential difficulties may have been overlooked. This paper reports a case study of a Wiki which was implemented to support a group of researchers. Although belonging to the same institution, the group members were relatively dispersed and their research areas were disparate. Nevertheless a short study showed that there were benefits to be gained from sharing knowledge and that many of the researchers felt that a Wiki would be a good mechanism to support this. A Wiki was implemented and was initially very successful. A significant number of researchers contributed to the Wiki and almost all made use of it. However the usage declined over time and attempts to stimulate interest by providing incentives for contributions were unsuccessful. One year after launch use was minimal. A qualitative study was carried out to understand the reasons for this decline in use, and is reported in this paper. Responses suggest that two factors may have been particularly significant in explaining the failure of the system. One problem appears to have been a lack of critical mass. Only a small proportion of users are likely to contribute and there may be a threshold size for a community to be able to support a vibrant Wiki. Time also seems to have been an issue. Some respondents said that they simply were too busy to contribute to or use the system. Organisations which are considering the use of Web 2.0 technologies to support a knowledge management initiative should consider the likely impact of these factors in their own situation. Although technologies such as Wiki have great potential there are also pitfalls in undertaking such projects which are not yet well understood.