Preferential attachment in the growth of social networks: the case of   Wikipedia
  We present an analysis of the statistical properties and growth of the free on-line encyclopedia Wikipedia. By describing topics by vertices and hyperlinks between them as edges, we can represent this encyclopedia as a directed graph. The topological properties of this graph are in close analogy with that of the World Wide Web, despite the very different growth mechanism. In particular we measure a scale--invariant distribution of the in-- and out-- degree and we are able to reproduce these features by means of a simple statistical model. As a major consequence, Wikipedia growth can be described by local rules such as the preferential attachment mechanism, though users can act globally on the network. 
A. Capocci, V.D.P. Servedio, F. Colaiori, L.S. Buriol, D. Donato, S.   Leonardi and G. Caldarelli