Considering all the technology Facebook has accrued directly and through partners a phone with a lot of nice features should be possible for them to implement. It's also a logical next step. Phones are social nodes in themselves, and mapping the Facebook 'world-in-a-world' onto this should be possible.
It could even be data-only if they wanted to (wifi/data traffic), but I don't think they would take it that far.
Technology mapping from the Facebook's technology chest to the mobile:
Text messaging - Replaced/complemented by Facebook Messenger
Audio chat - Integrated Skype version
Video chat - Integrated Skype version (Technology now in the hands of MS/Skype. Apple has shown us that this is feasible)
Group video chat / audio chat - Integrated Skype version.
Status of your friends reflected on your phone (Eg. approx. location, busy, last locations visited).
Contact list - Facebook friends.