What really sows how strong a grid is, is its level of social interaction. A grid that has too little social interaction will have the users feeling isolated (at that point there is little difference between an offline grid and an online grid).
Too few regions means there is not going to be enough variation in social interactions to hold the interests of users (not enough activity to bring them back).
With low population density and there is not enough people to interact with (feelings of isolation).
The more regions a grid has, the lower the population density. This means there needs to be a balance between the number of active users and the number of social regions (ie: regions used as social spaces rather than personal spaces).
Although not explicitly stated in these statistics, we can use them to estimate what the social interactions of the various grids are. A simple model would be to divide the number of regions by the active users. This would give us a population density (higher density would be better).
This is not a perfect model as there are many more factors that would apply (types of regions, total size of the grid, purpose of the grid, etc). Also, many grids have "hub" regions which are designed to concentrate social activity, while still allowing a relativity sparse population.
Virtual worlds are a social media and the strength of a grid is mostly dependent on the amount of social interaction going on. the more and varied the interactions, the stronger the grid is (and the more stable it will be as a business venture).