I'm surprised to hear someone as experienced as you say that. I've only been online since 1997.
All successful internet communities seem follow a common life cycle:
* Early adopters seem to be good
* They attract more users
* Someone pines for the old days
* Earnest discussions start about how to "save" the community
Here things bifurcate:
* Descent into infinitely recursive navel gazing with site population following a visible half-life; OR
* Equilibrium is reached after a certain number of the early adopters leave.
I imagine this can be modelled as stocks-and-flows. It would be interesting to see if there are any predictable tipping points or at least observable, predictive metrics.