It's really unreliable, though. Like I'm usually logged into facebook all the time on my computer so it might think that I've read something because it's come up on the screen, but if I'm not looking at the screen at the time, I haven't actually read it. Short of hijacking your webcam, facebook can't actually know that a person is looking at the screen at the same time as they are showing the message on it. What it should really say is "We have displayed this message on this person's screen."
Also I might be interacting on FB on my phone but the phone now requires a separate app for messages and I CBA to download it separately, the messaging bit on the browser screen is too small on mobile, so I usually ignore messages until I get home. But if they get dismissed, I might forget that I've got to reply to it.
AND if someone messages you on FB while you're doing something else, it flashes annoyingly in the tab which distracts my eye from what I'm doing. So I click on the message to make it stop flashing, but don't actually read it, sometimes minimise the message tab itself and then, again, often by the time I finish what I was doing before the message started rudely "shouting" (flashing: the visual equivalent) at me I've forgotten that I had a message, and I probably wasn't supposed to be on facebook anyway, so I don't get back to it.
Maybe people are ignoring you, but maybe they just don't revolve their lives around facebook messages. Just phone them if you want to talk to them urgently 