i tot white iphone is having delay ?

I would like to ask a question about the Data thingy.. let say i wanna on a msn-like application for 24/7.. does it takes up data per second or only when i use it? and do those applications have notification although we're not on the application? and about some apps that is free-sms worldwide such as WhatsApp.. does it take up much data? it will eat data every second or just when i send and receive? i dont know if here's the right place or not but if someone knows the answer please answer me question. thanks!