@atomicpoet

Could I suggest an alternate explanation?

People keep finding places to chat with other people, and then it keeps getting filled up with noise from various kinds of spammers, hostile actors, and just other loud people.

So they go somewhere else.

The migrations are less about technology and more about governance and not wanting to be interrupted with irrelevant shouting from others?

The technological phases are more about what technology happened to be popular at that time, but it's the digital equivalent of being in a painfully loud room that pushes people to move.