Most people would argue that they've not seen your messages, hence, the really long time taken to respond back to you. But these days, there are notifications that let senders know the message has been seen or read. The same thing is happening in Facebook Messenger, but here's a Chrome browser extension that will help you out.

Called Facebook Unseen, it blocks Facebook from knowing you've read a message so it doesn't get tagged with the "Seen" status.

You can download it here from the Chrome webstore.