Much of the current debates in the social service delivery have focused on the blurring boundaries between three sectors ? the nonprofit, business and public sector. Surprisingly no empirical research has been given to this phenomenon from macro and comparative perspectives. The study aims to bridge this knowledge gap by presenting the conceptual and methodological model to link organization and strategic management theory with network theory. Major findings uncovered by network analysis are; 1) Network structures are different according to specific types of collaboration relationships. 2) When three sector organizations are participated in the work referral network, the social service system emerges. Three sectors play a unique role respectively ? a sender for public agencies, a service provider for businesses. As a major actor in the social service field, nonprofits not only play these two roles, but also play a coordinating or broker role between three sectors. 3) When either of the business or public sector is introduced in the collaboration network, new network structures replace the network structure which is composed exclusively of nonprofits.