Cross-enterprise collaboration has emerged as a key survival factor in today's global markets. Semantic Web technologies are the basis to establish enterprise interoperability including data mediation support and automatic composition of services. Capabilities of services are semantically described and reasoning techniques support the discovery and selection of services at run-time. These technologies are commonly based on precisely defined enterprise ontologies. In contrast to Semantic Web technologies that cover interactions between (technical) services, human collaborations emerge based on social preferences. Social networks have become a mass phenomenon. The fundamental aspects of these networks are to manage personal contacts and to share profile information with friends. These principles are increasingly harnessed in businesses and professional environments. In a manner similar to service-oriented systems, they enable flexible discovery and dynamic collaborations between participants. In this paper, we discuss the concept of social overlays for Web service based collaboration infrastructures. This mechanism enables information flows between actors in order to allow for flexible group formations in highly dynamic large-scale networks.