Unique post-id for WordPress Multisite Network

Yes. Use $post->guid instead of $post->ID.

NB: guid must be in lowercaps for it to work.

Detailed explanation here: https://wordpress.stackexchange.com/a/209832/71131

Some additional detail on the purpose of keeping $post->ID as a backup in your third-party service: https://wordpress.stackexchange.com/a/209833/71131