A representative of Facebook confirmed for me that the email service handling mail for facebook.com email addresses is no more. PC Mag had previously shared that FB was warning users back in April that this was coming.
Thus, it is now safe to block, reject, unsubscribe or otherwise filter out all mail to the facebook.com domain. I have no clue what they'll do with it in the future, but if it was me, I'd let it bounce for a while, then turn it into a spam trap domain, feeding spam filters or blacklists. If they have the same idea, you'll definitely want to stay away.