Sent to you by Chris Hunter via Google Reader:
SMS fees are pure profit for the cellular carriers. They're basically free for carriers to send, but they can often cost ten cents or more per message. It costs more to send a text message on Earth than it does to transmit data from Mars.
Given these extortionate fees, it's no surprise that a variety of apps are springing up that allow people to send text messages for free and avoid the carriers. The most popular, WhatsApp, claims it has more users than Twitter and sends more messages than Facebook worldwide.