Facebook's app transmits calls using your phone's broadband connection, whatever it is -- 3G, 4G or Wi-Fi. Instead of sucking up your monthly allotted voice minutes from your cellular carrier, it sucks up megabytes from your data plan. That's a boon for heavy talkers with unlimited data plans.
This is nothing new or technologically groundbreaking: Skype was doing it long before Facebook. Facebook has many more active users than Skype, though, and the allure of having to use one less network or app will appeal to some (perhaps many).
But it's not all roses just yet. For now, this is a pretty limited offering.
Your phone won't alert you the way it does when you have a normal call: Facebook calls just show up as a push notification. And right now, you can only connect with a conversation partner who also has Messenger installed on their iPhone.
There's no way to ring up a Facebook friend who is logged in through the website, and there isn't any way to place a call to a ten-digit phone number. Calling is still absent from the Android and BlackBerry versions of the Messenger app.
There's a good reason for those omissions. The VoIP features in the Web version of Facebook were built using Skype's technologies, while the calling technology in Messenger is all Facebook. The ability to place and receive calls using traditional phone numbers requires telecommunications resources Facebook doesn't have (at the moment, at least).
For the time being, services such as Google(GOOG) Voice and Skype have a leg up, given their larger feature sets. But Facebook generally doesn't let anything remain in stasis for long. As we've seen in the past -- and caught a glimpse of yesterday with the Graph Search reveal -- Facebook likes to start small, and steadily flesh out its products. Expect the same with voice calling.
And unlike Google Voice and Skype (which is owned by Microsoft(MSFT)), Facebook is 100% platform neutral. Facebook(FB) doesn't care if you use its app on an iPhone, Android, Windows Phone, BlackBerry or something crazy like Ubuntu.
In a time when every company is saving the best features of its products for its own mobile ecosystem, Facebook has the opportunity to both usurp its direct competition and build out a telecom-slaying voice service.