On the other hand, I think it would be suicidal. While I agree that the phones of some (may be many) might be utilized solely for the purpose of checking Facebook regularly and on the move; there are still other reasons why people prefer one phone over other.

They have already opened up enough APIs to allow for decent third party clients - if they make a policy to withdraw it, developers and users who have already bought a smartphone will not only panic but probably revolt as well.

Alienating existing userbase, when alternative platforms like Quora are just starting to get very popular would be suicidal. In fact my university is already seeing a mass migration from Facebook to Quora and reddit (and HN).