I hate to be the devil's advocate but I swear by Facebook's messenger app. It does exactly the same things: international, FREE-as-in-beer, really cross platform (can whatsapp do web?), push notifications on iPhone thereby effectively replacing SMS, etc. As long as the other person is on facebook of course!

It has come to the point that my wife and I barely use SMS anymore and are actually saving some play money on SMS thanks to that thing.

And that is bad news for carriers worldwide yes, but not that anyone really feels pity for them anyway.

You see, so what stops them all from implementing their clients and servers using conformant XMPP with enabled federation? Can your Facebook messenger connect you to users of WhatsApp and vice versa? No. With Federated XMPP it'd just work. Is it just their greed, or lack of thinking? Actually Facebook does use XMPP, and I think even WhatsApp in some way internally does, but Facebook server lacks federation, and WhatsApp isn't even conformant to standard XMPP. In the end - you have no way to connect the two.

This situation resembles the early period of the Internet, when users of Compuserve couldn't send e-mails to users of AOL (and other way around). It sounds completely weird today, but how is this situation with IM networks different?

Understandably there are historic isolated networks (AOL, MSN etc.) which predate any serious federation efforts, and even they are slowly enabling XMPP in some ways. But creating new closed ones in the present time is just weird and only serves to make the situation worse.

Using Facebook's messenger app requires you to know someone in way which just having their phone number doesn't.

For example, if I'm going to go on a date with someone and we've swapped numbers that's enough for us to talk by SMS/iMessage/WhatsApp, whilst we probably[1] don't know each other well enough yet to expose our digital lives to each other on Facebook.

[1] - This entirely depends on things like age, social norms, how much personal information is on your Facebook page etc. Also, depending on are you going on a date with someone you already know vs. someone new. Mostly the point being that anything that just needs a phone number has a low barrier to communication while also not leaking any personal information (other than your phone number).