Don't know if I'd call a Facebook Phone an iPhone Killer. But maybe closer than anyone else has come.
Although it may not be of interest to many here, Facebook has a solid user-base and has even, in ways, built its own App Store. You'd have instant access to all your contacts, synced over FB, a built in, synced Calendar app, "free" messaging (i.e. FB chat used through push, using only data and not text messages) and quick photo and video sharing.
Throw in, say, iTunes integration to allow you to upload your iTunes music to your Facebook Phone and I'd say you've got a solid use case for many people, particularly younger people who essentially live their lives on Facebook and have never really known a world without it.
The Facebook Phone, if it exists, wouldn't be for us here; it'd be for our kids, or for people who've never really done the smartphone thing before. Price it right and you'll put a serious shot across BlackBerry's bow in terms of their marketing to youth with BBM.
My two cents, anyway.
i7 iMac / iPad 32GB WiFi + 3G / iPhone 4 (Black 32GB) / iPod nano (4GB Black 1G)
"What we call the beginning is often the end. And to make an end is to make a beginning. The end is where we start from." - T. S. Eliot