%T Preferential attachment in the growth of social networks: the internet encyclopedia Wikipedia %O ? 2006 American Physical Society %I American Physical Society %V 74 %K PACS: 89.75.Hc; 89.75.Da; 89.75.Fb %A Andrea Capocci %A Vito D. P. Servedio %A Francesca Colaiori %A Luciana Buriol %A Debora Donato %A Stefano Leonardi %A Guido Caldarelli %X 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 those 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, who are responsible of its evolution, can act globally on the network. %L eprints1109 %D 2006 %J Physical Review E %N 3 %R 10.1103/PhysRevE.74.036116