This model is incomplete at best. In my view "possums" here are not simply people obsessed with staying on topic, they are people who are deeply invested in the topic. Other terms for possums would include makers, scholars, artists, and founders. So called "otters" are those who have a passing interest in the topic, enough to be entertained by it but who do not participate at a deep level. They are otherwise known as consumers or dilettantes. There is a third group missing, they generally move in after the "otters" have reached a critical mass. To stick with the theme let's call them "orcas". They have no interest in socializing or in the topic, their goal is to monetize the topic and extract wealth from the otters by exploiting the work of the possums.