[Air-l] Community "Critical Mass"?

Hello Everybody, I've got a (hopefully) quick question: is anyone familiar with research into when an online community attains a "critical mass" and becomes self-sustaining? In terms of community, I'm thinking of online communities such as wikipedia, YouTube, MySpace, etc. that specifically require participatory involvement. In terms of "critical mass" I'm thinking of audience size / reach, ratio of "posters" to lurkers, etc. Does anyone know of research into this area? Any and all help would be greatly appreciated. All the best, Chris Modzelewski -- Chris Modzelewski Emerging Analysis Corporation