It is one of most popular apps around the globe, it has a lot of users and since Facebook acquired it, its popularity only growing. So what makes it so competitive among a lot of similar apps designed to communicate and chat using internet connection.
WhatsApp uses your phone number as your ID similar to Viber. So basically you are limited to only one device. You cannot have your chats on your desktop computer or on other phones. You have to have person’s phone number to be able to reach it even though it uses WiFi or 3g for connection. It is easy to install and its free for first year, after that you have to pay $0,99 subscription fee (while any other competitor does it for free: Viber, Skype, Facebook Messenger). There is a group conversation function. Also apart from calling you can send text, voice or video messages. It is nice location share function with little map included.
Pros: a lot of users; easy to use; messages are free;
Cons: It has $0,99 yearly subscription fee after first year of usage; You can only use on your phone;