Reengo allows free calls between Facebook friends who have Reengo installed over 3G and Wi-Fi, and runs efficiently without draining the iPhone's battery.
Reengo differentiates itself from most competitors in that it concentrates on a very simple function of calling based on the 'platform de-facto' Facebook, doesn't require phone numbers at all, and the very important fact that it doesn't need heavy infrastructure, making it easily scalable.
Facebook page
Official twitter account
CEO Yanasawa’s twitter account