This article describes various configurations of activities on Facebook. We rely on a quantitative survey over 15,145 Facebook users to propose a morphological and structural interpretation of user behavior on online social networks. Six configurations are identified from the traces of the various possible activities on Facebook: Non-active users, two configurations dominated by conversation (in groups or distributed on the pages of Facebook friends) and three classes of users who prefer to express themselves on their own page (egocentric, egovisibles, sharers). By combining these different configurations of activities with socio-demographic and structural indicators, we highlight the critical role played by of age, gender or friendship network structure on the prevalence of those configurations. We also exhibit a strong sensitivity of the network according to the age of user, and a clear specialization of a network of regular commenters among most active Facebook users.