Software systems are becoming more and more part of human life influencing organizational and social activities. This introduces the need of considering the design of a software system as an integral part of the organizational and social structure development. Alternative requirements and design models have to be evaluated and selected from a social perspective finding a right trade-off between the technical and social dimension. In this paper, we present a Tropos-based approach for requirements analysis, which adopts planning techniques for exploring the space of requirements alternatives and a number of social criteria for their evaluation. We describe the tool-supported analysis process with the help of a case study (the e-voting system), which is a part of a project funded by the Autonomous Province of Trento.