Have you tried viewing your iCloud keychain on macOS? I'm not sure if it's inclusive of entries made from iPhone-only apps, but there's definitely an option to view entries synced to iCloud for other things.

I think I tried that a long time ago, including various tricks to see the hidden folders on the iPhone file system, but it didn't work.

Now I'm not going to install any FB-related app on my new phone to test any other ways, because I'd rather not let them mark that device too if I can help it.