[a_imho]

You could send emails to FB users directly to their facebook email (don't know whether this exists anymore). In 99% these emails were relayed directly to the users mailbox (probably with the added benefit of coming from the facebook.com domain).

Many reported this, but it was not eligible for bug bounty, it was a feature according to FB, even though it circumvented their pay 1$ to deliver your message to someone you are not friend with.

[undisclos3d]

Emails sent to a user's Facebook email address goes to their messages (FB Messenger) mailbox, but they may receive a notification via their personal email.