I think Facebook should cozy up to Mozilla and use Boot2Gecko. Facebook's strength is in their web presence, and by leveraging a web-oriented phone, they could offer something that doesn't compete directly with other handset makers.
Mozilla and their leadership care very much about privacy and freedom. Facebook does not jive with that at all.
Furthermore, facebook might be somewhat evil when it comes to customer data, but they're an upstanding citizen in the open source community. they manage or contribute to many open source projects.