Washington Post: Facebook's Messenger app now allows U.S. users place free voice calls over WiFi.
To use the feature, Facebook users must hit the “i” info icon in the corner or a conversation or contact information page. That panel has a “Free Call” button that you can use if your friend has shared a mobile number with Facebook and is available for a call.
The company slowly has been building out the features available in chat — most notably with the 2011 Skype partnership that put video calling on the Web version of its site. When it released Facebook Messenger last fall, it became even clearer that messaging and mobile applications were priorities for the company.