Apple’s iMessage is killing off the overpriced text message, at least for a narrow group of iPhone owners.

Graphic designer Neven Mrgan posted this chart showing how his text messaging use changed after Apple released iOS 5 with iMessage. As you can see, it dropped dramatically.

Most of Mrgan’s friends are iPhone owners, so iMessaging is free between them. If you own an iPhone but most of your friends are on Android, RIM, or feature phones, you’re still going to have to pay for text messages.

Regardless, the writing is on the wall for text messaging. Carriers are going to be losing one of their easiest sources of profits.