> fascinating how often you hear these stories (and I experienced one, myself), of communities "moving to Facebook" and basically dissolving as a community

There's an analogy here to suffocating in an anoxic atmosphere.

Our bodies don't sense blood oxygen well. Instead, our urge to breathe is mainly driven by dissolved CO2 [1]. So if you're breathing out CO2, and breathing in no O2, your alarm bells stay mostly silent. Your lights go out without your ever being wiser.

Analogously, I think our social senses trigger when we've been away from people we care for. We get that "I haven't seen so and so in a while" urge, which in turn drives reaching out.

The problem is that sense seems almost like a proximity timer. If we've interacted in any way with so and so, it resets. A threshold which appears to be met by e.g. liking a photo on Facebook–empty calories of social interaction. A nitrogen atmosphere giving the perception of normalcy while everything slowly decays. And then, at a moment nobody notices until it's passed, the social rot sets in and a former community is now folks who once knew each other.

It petered out.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC4515048/#b21