The observation that many important factors increase more than linearly on the size of a network or group.
The NetworkEffect is normally cited as the reason for the ultimate demise of many closed information sources when faced with the open InterNet. Because the InterNet is several orders of magnitude larger than most closed information sources, unless a closed information source has features that the InterNet lacks (and those features are sufficiently important) then users will eventually use the InterNet rather then the closed information source.
When measuring the size of a group of humans the Hacker / Luser ratio can be important. A sufficiently low ratio threatens to drown the Hackers in Lusers, reducing their productivity unless specific steps are taken (see BugZilla). Low ratios are to be found in GUI applications, particular games and OfficeSuites communities (although it could be argued that lusers aren't part of the community). High ratios are to be found in CompilerCompiler and similar communities.
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