The facility could prove particularly popular with teenagers, helping them save money on their data plans. But how revolutionary is this move for Facebook really?
Potentially it could have serious implications for conventional mobile carriers if Facebook users simply bypass their mobile data plans. Android phones are not able to use this Facebook feature - yet.
"We started testing this in Canada the first week of the year," Facebook told the Telegraph, "and today we're extending that test to the US. We were able to expand the test so quickly because it went well in Canada and we wanted to expand the audience."
US iPhone users tap the 'i' on their profile followed by a free call button. The call is then routed through an existing wi-fi connection.
There are however several other alternatives to this service already. For example, Viber and
Skype offer similar free-call offerings through wi-fi networks, which are also Android-compatible.
Existing iPhone users can also use Apple's FaceTime video-chatting technology via wi-fi too. So Facebook's offering needs to be seen in the context of other competing service offerings.
At the time of writing (12.20pm, 18 Jan), Facebook shares were worth $30, some distance from its September drift under $20 but still way off its IPO offering price of $38 earlier last year.
Meanwhile Google's Larry Page has hit out at Facebook claiming, in an interview with Wired Magazine, that "they're doing a really bad job on their products".