if you have Unlimited messaging then you wont have to worry about any data charges at all. The IM messages will be classified as Text Messages on your bill.....(this is with AT@T but most other company's are close to if not the same) If you have a limit on texting that limit will also account for Instant Messages sent or received. Even though on a computer the messages are sent through the Internet to the other person, with a phone it doesn't use the Internet....the other persons computer uses the Internet to send it to the cell towers then the cell towers send it to your phone and vice versa. So no charges for using the Internet to send IM's!