That might be the case in your country... but in Latin America, for example, WhatsApp is HUGE. People don't text each other, they use WhatsApp. Mobile carriers provide plans with discounted traffic for WhatsApp, or sometimes even free. Entire family and friend groups constantly keep in touch using WhatsApp.
In fact, Bolsonaro used WhatsApp spam as part of his election campaign strategy[1].
In those United States, you would be right to think so. Yes, there are other chat apps. A flood? No. A (cess)pool, maybe.
Outside of WhatsApp (2 Billion Monthly Active Users), Messenger (1.4 Billion MAU) and WeChat (1.2 Billion MAU) the others stand little chance of building and benefiting from being a platform.
Setting aside WeChat and China, if you now look at the next billion users who are going to be using the internet for the first time, WhatsApp is the lens through which they see the internet - they communicate, consume news and entertainment, buy and sell things, get medical advice, even fall in love via WhatsApp.
In India, where a significant portion of that #nextbillion are gonna come from, Messenger has ~100M users and WhatsApp has ~500M MAU. Similar multiples play out in "WhatsApp first" markets like Latin America, most parts of Asia.
Case in Point: Even though it had nothing to do with our business, we recently helped hack together a solution to conduct two, 20-question, exams for 100K students in an Indian State. Simply because every family had WhatsApp (and nothing else). Messenger simply doesn't have the reach in the emerging world.
Why then? Why not any other chat app, or even messenger with far greater reach and just as many resources?