So I had a flick through the website, and I fail to understand how this service makes any money.

This automatically makes me suspicious that either they'll eventually start collecting data, or the service will disappear as soon as they are acquired.

Looks great, I was not aware they had open source client.

Given that they were forced to release their Axolotl (aka Signal protocol) under GPLv3+, they try to add some strange restrictions [2]:

"Additionally, if you choose to build an Open Source App, certain restrictions apply, as follows: a. You agree not to change the way the Open Source App connects and interacts with our servers ..."

As I understand I am free to ignore any additions to GPL.

[1] https://medium.com/@wireapp/axolotl-and-proteus-788519b186a7... [2] https://github.com/wireapp/wire

I also recommend Wire. I tried multiple alternatives for Skype and Wire was the only one which had good call quality and clients for all major platforms.