facebook login & register button missing

hi,

not sure what happen, awhile ago on login page i still able to see the facebook button to login.
at backend i went to see any setting to register by facebook name, but i can't find.
some how i did a save. on front end the facebook login went missing. even i redo auto tune again it still missing.

anyone have any idea how to get back the button ?

at autotune error check, all is ok