Sometimes I write messages to some chicks I don't know personally and they are no friends of friends of mine. Few minutes after I wrote a message, I see a "seen: 8:24" below my message. I can't believe that the receiver chick already read my message few minutes after I sent it. Another important information (for the next example) is, that this receiver is in the same Facebook group as me. I figured out that a receiver which is not a friend or a friend of friend of mine see my message as incoming mark on top bar when she is in the same group. Otherwise the receiver wouldn't see the message as incoming mark.
Another example: Some months ago I wrote another chick which was not in the same group as me so she probably never saw my message (because it's not marked as incoming). Despite that there was a "seen" at the same day. It's improbable that she really read my message. So what does "seen" exactly mean? Is it just a confirmation of receipt. I doubt that the receiver really read the message or opened it.