Vonage offers free calls to all your Facebook friends who, virally, have downloaded the same app. The calls work over WiFi and 3G/4G networks. Vonage works on iPhone and Android phones currently.
Once you install Vonage Mobile for Facebook, the app trolls your Facebook profile for compatible friends and shows you that they are able to either take voice calls or chat. Just like thousands of other Facebook viral apps, Vonage also constantly pester you to tell your friends about the app, thereby fulfilling its aim to form a daisy chain of Vonage network. I can already see some of you worrying about the privacy issues. If you are worried simply don’t install the app. Vonage is available at iTunes store and Android market and is currently aimed to work with iPhone, iPod Touch and Android phones. Here is a small video demo