At the international level, economic, political and civil society institutions have been recently resorting to social partnerships to build networks with several social actors. Partnerships often involve public agencies, private firms and non profit organizations into local development policies. Strengthening the societal role of civil society actors, they also promote the so-called subsidiary practices. Morever, in Italy there are normative and organizational limits for their implementation. In this perspective, partnerships should become an operative arrangement of welfare society, aiming to govern relations between social actors (market organizations, political organizations and civil society organizations) involved in generalized actions of social policies, especially at the local level. The paper introduces and discusses the results of two qualitative case studies concerning social partnerships established in Lombardia, also by local community foundations, assuming that they are “intermediary trustee”. The partnerships case studies test if, and how, the community foundations produce and/or increase social capital of a local community collaborating with other partners. The paper focuses on inter-organizational trust as an important issue for partnership functioning, trying to give an interpretation of its emergence moving from the rational choice and the gift paradigms.