I never understood the whole Whatsapp/tango/viber craze. These features are built into every iPhone. If you are on wifi iMessage and FaceTime (video and audio) are free. No need to download any app. The only situation is if the other person doesn't have an iPhone. If they don't and I am away and it is really necessary to get in touch with them I will email them or spend the $2/min to call them.
I use Telegram a lot because it's cross platform and runs on EVERYTHING.
You can Telegram on an iPhone, on a Windows PC, on a Mac, on an Android phone, on a Linux box.. ANYTHING.
iMessage only works on iOS devices and Macs.
Text messages don't work well on desktop computers.
The simple truth is these apps make it easier for a wide variety of people to communicate. Not everyone has (or should have!) an iPhone. People like different things. Let them enjoy them.