"FaceTime on iPod touch makes it possible. FaceTime works right out of the box — just enter your Apple ID and email address. Or create a new email account just for FaceTime. Using FaceTime is as easy as it gets. Say you want to start a video call with your best friend over Wi-Fi. Just tap the FaceTime app and find her entry to start the call. An invitation pops up on her iPod touch or iPhone 4 screen asking if she wants to join you. When she accepts, FaceTime begins."

Is this means the Facetime like an instant messaging? But since it is using WiFi, what is the limitation of it?