i know this is quite old - but as the question (at least to me) is still quite up-to date your thoughts 2019 would be interesting.
Friends of mine use whatsapp, threema, telegram and viber - i tried out riot/matrix (on sailfish (sorry) - thinking about pinephone as next devices)). Also i tried delta chat. Threema and whatsapp are most used.
Problem is to find common ground - as i first of all don't want my phone book taken and second most dont see the point in switching (entering more then a phone number to start is soooo hard, and also no one uses it). Thats why i tried delta chat - but not sure so far.
Anyways - as i am new to ubuntu touch i would like to know what messengers (prefered the "full package" - meaning, chat, calls, video) work or are planned with this os?