I want to deactivate my Facebook account, but I use the Facebook login for a number of apps and games external to Facebook. Will sign on for these other apps fail if I deactivate my Facebook account (and don't delete it)?
Once you deactivate your Facebook account, you will not be able to login any app or game which is associated with your account. Deactivating account means hiding your account temporarily and it will not delete any information even not game scores etc.
When you reactivate your account, all photos, games etc. will come back within few hours.