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Diversity is an important characterization aspect for online social networks that usually denotes the homogeneity of a network's content and structure. This paper addresses the fundamental question of diversity evolution in large-scale online communities over time. In doing so, we study different established notions of network diversity, based on paths in the network, degree distributions, eigenvalues, cycle distributions, and control models. This leads to five appropriate characteristic network statistics that capture corresponding aspects of network diversity: effective diameter, Gini coefficient, fractional network rank, weighted spectral distribution, and number of driver nodes of a network. Consequently, we present and discuss comprehensive experiments with a broad range of directed, undirected, and bipartite networks from several different network categories -including hyperlink, interaction, and social networks. An important general observation is that network diversity shrinks over time. From the conceptual perspective, our work generalizes previous work on shrinking network diameters, putting it in the context of network diversity. We explain our observations by means of established network models and introduce the novel notion of eigenvalue centrality preferential attachment.

Abstract. Managing one's memberships in different online communities increasingly becomes a cumbersome task. This is due to the increasing number of communities in which users participate and in which they share information with different groups of people like colleagues, sports clubs, groups with specific interests, family, friends, and others. These groups use different platforms to perform their tasks such as collaborative creation of documents, sharing of documents and media, conducting polls, and others. Thus, the groups are scattered and distributed over multiple community platforms that each require a distinct user account and management of the group. In this paper, we present dg FOAF, an approach for distributed group management based on the well known Friend-of-a-Friend (FOAF) vocabulary. Our dg FOAF approach is independent of the concrete community platforms we find today and needs no central server. It allows for defining communities across multiple systems and alleviates the community administration task. Applications of dg FOAF range from access restriction to trust support based on community membership.

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Web communities court for the users' favor. Maintaining a high quality of the user-generated content is a foundation of a healthy and flourishing community. In order to maintain this quality, the community platform needs governance. Governance of a web community can be understood as steering and coordinating the activities of community members. This includes viewing and reviewing of already existing content but also limitations for creating and sharing new content. In this paper, we systematically review successful web communities for creating and sharing user-generated content. To this end, we have grouped the communities into the following four categories: Social Media, News, Social Reviewing, and Social Networking, we have chosen the five most prominent web platforms from Alexa Page Rank. For our survey of these platforms, we have analyzed the functionality they offer for governing the user-generated content. The result is a description of best practices for governance of user-generated content in different categories of web communities.