A couple days ago Facebook held an event where speculation was that they would announce a "Facebook phone". We know that the announcement ended up being their new Graph Search functionality and I have to say I personally never believed there would be a Facebook phone announcement and think that's a dead end. However, last night Facebook did release an update to Facebook Messenger for the iPhone that added free calling via WiFi between Facebook iOS users. You can already make free WiFi calls via other clients, such as Skype and Truphone, but Facebook has more users so there is the potential that this could be more important for Facebook users.
As you can see in my screenshot, the updated Facebook Messenger for iOS app adds a new Free Call button when you tap the "i" icon in the upper right of a conversation in Messenger. I discovered this updated application late last night and since none of my Facebook friends were up I couldn't test it out myself. According to the folks at The Verge, call quality is good and on par with other clients.
We have seen companies like HTC try to create a Facebook phone and Facebook has apps for every mobile platform. I don't see why Facebook would waste time trying to make its own phone and think we will just continue to see improvements in Facebook apps for iOS, Android, Windows Phone, and BlackBerry 10. CNET put together some good reasons it makes little sense for Facebook to develop its own phone.