Hello Tyr,
Thank you for commenting.
Let me clarify that the 90 usually does not include those orphan accounts or those who ask one question and left. Those people are really transient visitors who are not really part of the community (even they have an account). The 90 is really the lurkers, who belongs to the community by virtue of repeatedly consuming the community content, just do not participate.
That being said, a lot of people who analyze this simple take the registered user with no post as the 90 because it is simpler. But that is not correct! You must take a look at users who have login within a certain time period to assess whether they are still part of the community.
Moreover, lurker include those who repeatedly visit the community for content, but never even register. Getting an accurate estimate of lurker population involves several data sources that need to be distilled. And we have all that data, we just need to analyze it and make it presentable. So it is more difficult. And when I get around to crunching those data, I will post the result.
This post, however, only focuses on the 9-1 part of the 90-9-1 rule as you've mentioned.