There are, of course, a few caveats:It only works on iPhone. It only works in the U.S. and Canada. Both parties need the separate Facebook Messenger app to make calls. It won't work with the regular Facebook app that you use to update your status and check your News Feed.
But the calling feature in Messenger hints at the grander plans Facebook has for you phone. Just like Facebook is increasingly doing everything in its power to keep you stuck on Facebook.com on your desktop (the addition of search last week is a big step), it's becoming clear that it's also doing its best to keep you within its app ecosystem on your smartphone, even when making calls.