Web service composition is the process of synthesizing a new composite service using a set of available Web services in order to satisfy a client request that cannot be treated by any available Web services. The Web services space is a dynamic environment characterized by a huge number of elements. Furthermore, many Web services are offering similar functionalities. In this paper we propose a model for Web service composition designed to address the scale effect and the redundancy issue. The Web services space is represented by a two-layered network architecture. A concrete similarity network layer organizes the Web services operations into communities of functionally similar operations. An abstract interaction network layer represents the composition relationships between the sets of communities. Composition synthesis is performed by a two-phased graph search algorithm. First, the interaction network is mined in order to discover abstract solutions to the request goal. Then, the abstract compositions are instantiated with concrete operations selected from the similarity network. This strategy allows an efficient exploration of the Web services space. Furthermore, operations grouped in a community can be easily substituted if necessary during the composition's synthesis's process.