Dr. Michael Wu, Ph.D. did a detailed analysis of the social media rule of thumb 90-9-1. This rule says that of the people participating in social media 90% are lurker, or just view material. Nine percent (9%) occasionally contribute. And only one percent (1%) participate often.
Rules of thumb are great to have and extremely useful to give a reasonable measure without getting bogged down. However, it is worth validating these rules.
In the post, The 90-9-1 Rule in Reality Dr. Wu detail the statistics from over 100 communities to show exactly the reality of the rule.