I'm not very experienced with FB but I use it to chat to a couple of friends as it's easier than email. Sometimes I get a notification that my last message was read at a particular time. Recently I've sent messages to three new people I've recently got in touch with after many years and I see no sign that they've read them and got no reply to any of them. At the top of each of my unanswered posts is a little left pointing curved arrow. They've all accepted the friend request so I know they saw that but no evidence they also saw the message.
I'm now beginning to think that the "read" notifications I get are only when my messages are read on a mobile device like a phone. Maybe if someone reads a message from a pc I should not expect to be able to know if they have actually read it or not. I do have notifications turned on for all people.
Can anyone clarify?