You can't call keeping merely the contacts without emails themselves "migration of email provider". It makes no sense.
Also, you said "follower[s] don't have to do anything", but somehow you (the followee) on the other hand needs to actively move? What if my follower is on this instance too and they don't actively move? Shouldn't their account disappear (and you lost your follower)? I genuinely don't understand how it would work other than everyone has to manually move together.
That's mistaking the map for the terrain. It's a networked system for leaving and retrieving messages. It's not a group of friends. The way you expect to migrate a messaging system is by moving the messages.
To confirm, I wish they would migrate posts, too, but I do not believe that the lack of that means that you cannot call it a migration.
However, your definition seems overly pedantic? It defines itself[0] as a social network with an emphasis on audience. Messaging is merely the method of interaction.
[Edit] "audience" is incorrect, I should've said "people"
A "social network" is a networked system for leaving and retrieving messages. Again, it is not a group of friends. It is a messaging system for a group of friends, just like a map is a graphical system for navigating a piece of terrain.
I'm not the person who said that but I can take a swing at it:
If the person does not migrate off of the instance, they'll lose the account and yes, you'll lose a follower. But if they do migrate, both of you keep the connection.