Too many topics, too little time

Participation Inequality: Lurkers vs. Contributors in Internet Communities (Jakob Nielsen’s Alertbox):
All large-scale, multi-user communities and online social networks that rely on users to contribute content or build services share one property: most users don’t participate very much. Often, they simply lurk in the background.

In contrast, a tiny minority of users usually accounts for a disproportionately large amount of the content and other system activity. This phenomenon of participation inequality was first studied in depth by Will Hill in the early ’90s, when he worked down the hall from me at Bell Communications Research (see references below).
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it is important to realize that when we are talking about user participation and contribution, we are in fact talking about a very small number of people who have the time and motivation to contribute to our projects. frequently… system creators assume that their users will be just as enthusiastic as they are in creating things in their system their system, this is rarely true, but Nielsen points out some ways you might increase the number.