Facebook has begun rolling out a new feature that lets iPhone users place free calls. Users of the Messenger for iPhone app have long been able to send messages among themselves; following a successful test in Canada of a free calling feature for that app, Facebook is now offering U.S. users the same possibility. Calls can be made over Wi-Fi or on cellular data. The Verge appears to have reported on it first.

The move throws Facebook into the Voice over IP in a big way. Recall that Facebook has a billion users; Skype has a fraction thereof. Facebook, though, has already teamed up with Skype for video calls. Will Facebook’s move throw down a gauntlet to Skype, and by extension to long-standing Facebook ally Microsoft (see “Facebook and Microsoft: An IP Bromance”), which owns Skype?

CNET goes further, saying that Facebook “is potentially a bona fide threat to carriers.” I’ll confess that because of my recent experimentation in connectivity options, the thought has recently crossed my mind as to whether I even need a voice plan, since the bulk of my communication happens via text, email, or Facebook these days–and since VoIP is getting better and better. CNET also sees the product as a gambit for the teen vote, since teens often have limited or shared voice plans.

Forbes sees this as part of Facebook’s mobile monetization plan, long a source of worry for investors (see, e.g., “Should Facebook Buy Opera?”). So long as Facebook can keep users trapped in apps of its own, that’s time they can serve up mobile ads and keep users away from competitors (see “The Facebook Phone Is in Your Pocket”).

Lastly, this could ultimately be another play to do what Facebook essentially does best: mine marketable data. If Facebook knows who you call, it knows who matters most to you. Facebook has waded into these waters a little by asking you to separate close friends from acquaintances, but there’s ultimately no replacement for a plain and simple log of who you’re actually bothering to ring up. “It could be a killer set of data,” as TechCrunch emphasized years ago of mobile contacts.

Maybe there’s not some secret plot, however. Maybe this is just a case of Facebook taking the next logical step of its stated mission: to help the world connect. Still, it will be interesting to see if we increasingly think of Facebook not necessarily as a website or app or digital forum, but as a one-stop shop for all our communications needs. It will be interesting, too, to see how Facebook fares in this foray into what is reportedly a $15 billion industry.