Lock it on FB, don't have it remember your password. Email is abut trickier.
Also, don't leave your phone justifying around lol
Seriously though, there are apps in the app store that allow you to lock certain apps.
Not totally on topic but I also use that text a pin feature to protect my Facebook from being hacked without access to my cell number. It's not all that cumbersome since I always have my phone. It also becomes my warning that someone is trying to access my account. Kind of wish I had this two part security on my other public and private accounts.