Web 2.0 services are characterized by the fact that their value increases the more people are using it. A traditional static Web site does not improve" when visited by large amounts of surfers since it presents its content the same, static way. In contrast, Web 2.0 sites use information provided by the visitors explicitly (user contributions build up the site or part of it) or implicitly (user activities on the site are used for adapting its content or presentation).