In most online communities, 90% of users are lurker who never contribute, 9% of users contribute a little, and 1% of users account for almost all the actions that occur within the online community.
What is the 1% Rule? (Charles Arthur)
It is an emerging rule of thumb that suggests that if you get a group of 100 people online, then only one individual will actually create content, ten individuals will interact (commenting or offering improvements) and the other 89 people will just view the community.
What Marketers Are Missing About the New Lurker Demographic (Jack Holt)
Twitter is making a strong play to encourage its social "lurkers" to get involved--that is, the 40 percent of users who have an active account(s), but fail to tweet.