When Facebook bought WhatsApp, many started to look for alternatives. Telegram was chosen by millions of new users, probably also because it’s free to use. The problem with things free to use on the internet is this: If something’s free on the internet, you are the product.
This seems to be true yet again – Telegram uploads your full address book to the operators servers, of course without asking for your permission first. This is even more critical when one of the creators of Telegram is VK, the Russian Facebook alternative.
Contrary to that, the swiss based Threema asks for your permission to upload your address book and it’s used for synchronization purposes only. Even if uploaded, only aliases are transmitted.
Only Teleram and Threema provide useful end-to-end encryption, but on Telegram it has to be enabled explicitly (Secret Chat), while on Threema you can’t even disable it.
One constraint is left: none of the tested app is open source.