There are so many factors that can combine to influence this rule, and it's hard to generalize or even predict which ones will impact it the most. I co-founded

There are so many factors that can combine to influence this rule, and it's hard to generalize or even predict which ones will impact it the most. I co-founded a company that built and managed private online communities for large brands, and the fact that the sites were private/invite-only and facilitated meant the engagement levels were often much higher than 90-9-1. Here are some of the factors that we noticed could change the ratio for our clients - private/public forums, ease of content creation, whether the site is moderated, anonymous accounts, commonalities between members, site usability and the difficulty of registration (setting higher barriers to register can actually create higher engagement). Your mileage will likely vary...