Hey Michael,
I’m running a WordPress multsite network.
I installed the plugin, network activated, and added my Netmix.com App ID and Secret Key and still got the message: Add App ID and Secret on admin to make this plugin work
So, I registered another App ID and Secret for Netmix.co, which is the primary site in the network, and it still displays that message.
I also network deactivated the plugin and only activated it on Netmix.com to test to see if that was the issue and I still get the message.
Not sure why this is happening.
https://wordpress.org/plugins/wp-embed-facebook/