Facebook Messenger, developed by Crisp App, is a recent inclusion in the App Store which helps you make Voice over IP (VoIP) calls to your Facebook friends even if they are on browser. The app is built over Facebook’s API to connect to the Facebook servers, however, the VoIP feature has been developed by Crisp App Limited. Compatible with your iPhone, iPod touch and iPad, Facebook Messenger requires you to have a minimum of iOS 3.1.2.