Facebook may not be looking at building a mobile phone but a mobile platform instead based on the fact that two Facebook employees working on the project are Joe Hewitt and Matthew Papakipos, two top-notch mobile developers.
Both have immense expertise in resource-light operating systems as mentioned by Michael Arrington from Techcrunch; Arrington also noted that the former left the iPhone platform "in disgust" while Papkipos worked on Chrome OS before leaving in June 2010.
Facebook already has built a platform that has gone beyond its core original function; Facebook Credits for example allows people to perform commercial transactions while Facebook Connect enables other websites to communicate and transfer user data seamlessly with the social network.
Dan Frommer from Business Insider actually hit the nail on the head when he says that Facebook might even build one based on Android OS (which would be slightly ironic) before mentioning the name of a third ex-Googler, who joined the social network.
Whether Facebook wants to build a mobile platform using Google's help might be important but what's even more critical is how to convince phone manufacturers and network carriers to use it. Will Facebook actually pay them or give them a cut of all profits like in the case of the App Store?