Facebook post bout how to enable a form of two factor authentication via SMS. This system requires you to have your phone with you if you are accessing facebook from a new machine.
encryption, as a preventive measure, assumes the primary threat is coming from the "outside" - typically (but not exclusively), in the form of a hacker trying to intercept communications or extract data from a lost or stolen device. While such threats are real and you absolutely must guard against them, we've reached a crucial point in the evolution of mobile technology, and just as importantly, user behavior where the primary mobile security threat is no longer the faceless and malicious hacker, but instead the legitimate, fully authenticated owner of the device itself.