updated 06:41 pm EST, Fri February 22, 2013
Formerly a Messenger-only feature, still no Android version
Facebook users in the US and Canada who are online with the service can now call each other for free on the main Facebook iOS app, a feature formerly restricted to the Facebook Messenger program. The feature, which works over both Wi-Fi and 3G/LTE connections, allows friends to start a chat session and then change it into a voice call. Currently the feature works with the iPhone or iPod Touch version of the Facebook app only, but should arrive to the iPad version through an update expected soon.
For some, the Facebook service may replace similar offerings from Google (through its GMail and Google+ services), Skype, Vonage, Viber and others. Like the Google option, however, both the caller and the recipient must be online with the service at the time, available for chat and accept a chat request before the call can be placed.
Skype and Vonage (and their competitor VOIP services) offer both free and paid options, with the latter able to be used to call real cell phones or "landline" telephones. Likewise, user-to-user calls on "true" VOIP services do not require that the program be open or the user logged in to be notified of calls.
Facebook Messenger had originated the feature a month ago in a likely response to the addition of free calling to Google+, but originally tested the option only in Canada, adding US service a couple of weeks later. Currently there is still no option for free calling in the Android version of the app, and Facebook has not announced any timetable for adding the feature.