It's huge in many countries where Internet service has been expensive or difficult to come by. Case in point, all my relatives in Eastern Europe use WhatsApp, and with better bandwidth options, some are using FB Messenger for video calls,.as well.

Facebook used to have (has?) 2G Tuesdays, where devs had to test their stuff at 2G speeds, for this exact reason.

Everyone I know in Belgium/The Netherlands/... uses Whatsapp. It's even got into the language: "to app someone", i.e. "iemand appen".