At a cost that provides long term interopraibility benefits. But these just care about short term rip offs and as well causing interoperability problems for their users (i.e. preventing communication with other services). We must be extremely lucky this didn't stay like that for e-mail.
> they gain nothing by just being yet-another-protocol.
That's the point. They measure their gain in how much they can mess up their users (by preventing them from communicating with users of other services). While gain should be measured in how useful such services can become (enhancing, not crippling communication).
> That being said, Facebook does let you chat via Jabber
It's not federated (which defeats the main purpose).
> I use it (with Messenger on OSX), and it works, but it's a completely mediocre experience devoid of what makes FB Messenger special.
If they wanted to improve things and thought that XMPP can't reach that goal, they could propose their non XMPP chat as IETF standard. Same as Google could do with Hangouts and so on.
