As expected, Facebook (NASDAQ:FB) has added a free calling feature to its Messenger app for Apple (NASDAQ:AAPL) iOS devices. It’s available only to users in the U.S. and Canada — so far. The calls can be made over WiFi or data connections.

The mobile voice market has become a commodity. Even the major carriers like AT&T (NYSE:T) and Verizon (NYSE:VZ) mostly care about selling data plans nowadays.

However, the Messenger service could be a serious threat to VoIP operators, such as Microsoft’s (NASDAQ:MSFT) Skype and Vonage (NASDAQ:VG). Facebook is likely to get substantial uptake from its massive user base, who certainly like to chat with friends!

And yes, Messenger is a highly popular app, ranked No. 17 on Apple’s App Store. With free calling, it’s a good bet that downloads will get a boost.