Update (November 2017): Unfortunately, Facebook has deprecated Comment Mirroring and it is no longer possible to use this feature: https://developers.facebook.com/blog/post/2017/11/07/changes-developer-offerings/.
We recently migrated a client to Facebook comments and enabled Comment Mirroring. Though the process is simple, it’s poorly documented and a little finicky. Here's a rundown!
Here's an overview of what's involved: