Replace
buildConfigField "long", "FACEBOOK_APP_ID", FACEBOOK_APP_ID
with
resValue "string", "FACEBOOK_APP_ID", FACEBOOK_APP_ID
then rebuild your project (Android Studio -> Build -> Rebuild Project).
The two commands both produce generated values – consisting of Java constants in the first case, and Android resources in the second – during project builds, but the second method will generate a string resource value that can be accessed using the @string/FACEBOOK_APP_ID syntax. This means it can be used in the manifest as well as in code.