A real, live Facebook phone is, in fact, under development, according to AllThingsD, and it's called Buffy.
They also report that the phone is 12-18 months away from being available. Literally, the world could have an iPhone 6 available by then. And Amazon is rumored to be cranking up a mobile phone, too.
So why the long wait? HTC is developing the phone, supposedly, with a customized version of Android. It takes time to develop custom anything, no matter how big a company is, but 18 months? Perhaps sexy new features are being developed (while Timelines are still in limbo due to legal issues).
U.S. & World
How the new phone/software/platform plays out on HTML 5 will factor in, too, as that language gains more and more ground on mobile devices.
Creating one's own walled garden, like Apple's, may seem appealing -- especially given their amazing per-unit margins and robust app store.
But, as Mark Zuckerberg stresses at Facebook events, it's a platform, not merely a social network. A platform where the 'best of breed' can show off their stuff. Sort of like an app. And maybe that's where Facebook should hang their hat, and not on it's own handheld device.