The 2 different API-keys for facebook-messaging (previously located in bundle c.o.messaging.facebook at /conf/ and facebook-contacts (previously located in bundle c.o.subscribe.crawler /conf/crawlers/facebook.yml) are consolidated into one:
This API-key is found / needs to be set in bundle om.openexchange.oauth.facebook at /conf/
The two parameters are com.openexchange.facebook.apiKey com.openexchange.facebook.secretKey.

These are set to the key for the domain per default and need to be set individually for each servers domain. The creation of the necessary facebook-application is documented at (facebook account required).
Please make sure that the applications settings includes the servers domain as Site