In essence, the network effect describes the positive externalities (value) of a product, service, or activity as more people use it. An organization taking advantage of the principle may refer to the practice as “crowdsourcing” (e.g. Wikipedia, Dell’s Ideastorm, iPhone Apps) taking advantage of the “wisdom of the crowds”. Individuals may also aggregate and mobilize for a specific cause, be it political, civic or commercial (e.g. Moveon.org, Ukrainian orange revolution). The emergence of the latter can be of spontaneous and real-time nature.
via hir.harvard.edu