At first it was just an sms replacement. Then people started using the group chats for (mostly) silly stuff or family groups. Then the group chat function started to be used more functionally for work and, say, flatmate-type stuff coordination and communication.
I wouldn't be surprised that if (or perhaps when) WhatsApp adds more functionality, it will quickly be picked up by everyone around me. I'm mostly surprised what WhatsApp hasn't done this already, especially considering that Facebook already offers a number of functions that I'd love to see integrated (to a limited degree) into whatsapp's paradigm (event creation/planning, more advanced photo sharing, 'blogging').
Does anyone have any idea why WhatsApp isn't becoming more like WeChat?