The Facebook features on Disney Emoji Blitz can only be used by players aged 13 and over. The Facebook option will not be available if you entered your age as 12 or under when first launching Disney Emoji Blitz.
NOTE: If you had entered your age as 13 or older, the Facebook login option can be found in the "Settings" (gear) menu under "About". See "How do I save my progress in Disney Emoji Blitz?" for detailed steps.