The development and usage of complex information systems leads to both technical and human challenges as large numbers of stakeholders with conflicting requirements are involved. Though global consistency must be preserved at the corporate level, software applications have to be adapted to particular needs. However, while individual users have the best knowledge about how to perform their job, they have little influence on the corporate information system. Our experience in industry shows that the "Knowledge/Influence Mismatch" leads to the "Information System Fragmentation" problem: departments, teams or individuals unsatisfied by corporate information systems tend to develop "parallel" ad-hoc applications. To cope with this problem this paper proposes core concepts for Social Information Systems in which much more power will be given to user communities, allowing them to extend existing applications for their particular needs, but also to share these extensions with colleagues; and this in a social way. This approach, based on the notion of "perspectives" linked to people, opens the possibility of a social adaptation and democratic evolution of the information system.