Everybody I know uses WhatsApp.
And even if I could convince them all to switch to Telegram, it’d only be a matter of time before somebody convinced me Telegram’s actually no good either—and which of the myriad messaging app alternatives would I need to convince 50 of my closest friends to switch to then? 
Quick reply: I’ve been using XMPP/Jabber and got quite a few people to join, using the Conversations app.
Advantage: it’s decentralised, so if you don’t like how a server or app is behaving, you can switch to a new one and keep all your contacts. You could even spin up your own server, if you’re that enthusiastic.
Disadvantage: all the options can be confusing, so you’d better choose one set (server and client) to refer all your friends to (and maybe set up their accounts for them).
There’s also a spinoff of Conversations called Quicksy which lets users sign up using their phone number, which might make it easier to get some people onboard.
(I have to go now but can come back when I have more time, if you want details)
One day it’ll be User A inputs a message into any of the XX top messenger apps, another technology encrypts and directs the message to any of User B’s preferred messenger app. One messenger interface per device.
