Facebook divides messages into two inbox sections - one contains messages from your friends. You know that one. Facebook displays small red number when you have unread messages in that folder.
Then there's the other one. It's been even called "secret inbox" on the internet and such, because it's very existence is not apparent. The inbox is called Message Requests and does not cause any notifications on Facebook. This can be very impractical, for example, if a member of a group you're in messages you and you notice few weeks later.
Is there a way to make Facebook raise notification if somebody who's not in my contacts sends me a message?