It is possible for you to read a message from others, without them knowing you did it.
While WhatsApp and Viber offer an option to disable sending "seen" status, Facebook doesn't have it. So here are a few tricks which you can do.
Google Chrome Extension
There are a few Goolge Chrome Extensions which block "seen" status, and allow you to peacefully read sender's messages.
One of them is simply called "Unseen". All you have to do is download the extension, install it and you're free to go.
Airplane Mode
You've got a message, it's killing you to know what is it all about, but you don't want to respond immediately or you don't want for sender to see that you've read it?
Activate "airplane mode", open the application, read the message then turn the app off. You can't just leave the app open because the sender will see that you've read the message as soon as you turn off "airplane mode".
We suggest you turn off the app, because all that wasn't worth it. :)