I think it depends on a country. Most of the people of my country and Russia use Telegram App. Because it's more convenient to use for us than WhatsApp.
However, my international friends always ask if i have a WhatsApp.. It seems many people are still using it. But in Japan I heard Line is quite popular. In China everyone uses WeChat
What's App is popular in Asia, maybe not so much in the Americas. FB bought it so not sure what is going to happen to it. I do like it. Has video, send pictures and send videos and text. What is necessary is that it has to have your telephone number so in that regards it will have who you are.