The overview page for any sent broadcast has a "Share" button. This gives you a permalink that you can share on FB/wherever. Just note that if you're using any sort of Liquid templating to do stuff based on subscriber data, the "subscriber" object won't be there – so ensure you have sane defaults (i.e. if you always include the first name of a person and 100% of your contact database has a first name set, the shared email wouldn't obviously have a first name – so ensure you have a fallback)
