Agreed, in Australia facebook prety much has a 95% monopoly on messaging, with iMessage a distant second. I haven't even heard of anyone using a Google product for messaging, just a few isolated instances of someone using Viber or Whatsapp (but nothing like how popular it is in Europe).

The primary advantage Facebook has, I believe, other than that everyone is already using it, is that you can find someone on it later without having to ask them for contact details.

Nobody would pay for a messaging service. Even if you would, it wouldn't matter, your non-tech-industry friends wouldn't.