Very interesting analysis. While "crude" it is a terrific starting point. I think you are spot on when choosing number of posts. Another term for that is activity and that is what parlays into the network effect.

While not exact, it is logical to think substantially more posts requires a great deal more people (nodes) which is the epitome of the network effect.

It will be interesting to watch the correlation you created over the next year or so to see how things change.