[ghaff]

Is iMessage a "Number-independent interpersonal communication services (e.g., messengers)"?

It's a messenger but it's based on phone numbers AFAIK--unlike something like WhatsApp.

[frumper]

You can sign up and use an email for iMessage through wifi

[ghaff]

Ah. I've only used it as a default SMS alternative on Apple devices including iPhone.

[rdsnsca]

It is, I use it from my Mac Mini without owing an iPhone.