Connecting client to server in chat apps like Viber or Telegram

I want to create an application like Viber or WhatsApp etc. My main challenge is connecting clients and server. Right now what i know that i have to use Websockets standard for connections. For web client i want to use standard Websockets and for Android and iOS i will use libraries for them based on Websockets. For example here you can see an example: http://ift.tt/1ovy1Fx

is this approach for this purpose is right? Whats other alternatives? For example Viber or Telegram how face this issue? Please help me to choose right approach before start. I will have 3 clients: Web, Android, iOS