This paper presents a simulation model for social systems which is based on individual interactions. A social organization can be considered as an intelligent information processing system. It appears that social systems involve a duality in their functioning, depending on whether the information exchanged between system components are rational or relational. Applying this point of view to an Employment Agency, we point out that the performances of this institution can fluctuate. This result echoes research works in the area of Conventions Economy and sociological researches on social networks. As a matter of fact, new prospects can be considered for the behavior of the labour market and new ways can be considered to organize an Employment Agency.