extrait du très instructif geekipedia, recueil non exhaustif des dernières tendances technoïsantes compilées par wired, et qu’il convient de retenir pour briller au prochain dîner :

‘’ Why did eBay become a giant, while other online retailers fizzled? In part because the auctioneer took advantage of network effects, or as economists put it, network externalities: the tendency for products or services to become more valuable as more people use them. Think of instant messaging. You probably don't base your decision to use AIM, MSN Messenger, or Yahoo Messenger on their features but on how many friends you have in each network.

This notion finds mathematical expression in Metcalfe's law. Named for Ethernet inventor Bob Metcalfe, it states that the value of a communications network rises in proportion to the square of the number of people who use it. But a network can take many forms. The near-monopoly enjoyed by Microsoft Office, responsible for a hefty share of the software behemoth's cash flow, is a function of network effects; when you upgrade to Office 2007, you're really buying continued compatibility with the worldwide brotherhood of Word and Excel users. Or take Nielsen's TV ratings: Networks rely on them not for their accuracy but because everyone else does. Even if the numbers are wrong, competitors will be off by the same amount.‘’