For those of you that use Facebook, another sneaky feature I use relatively frequently for messaging is the "Mark As Unread" feature.
If you happen to be wanting to view a message that someone sent you, but you don't want them to know you viewed it, you can open up the message, read it, and then click "Actions (top right corner) -> Mark As Unread" and it will remove the receipt so that the other person won't know that you've viewed the message. I've only tested this in the browser version of Facebook, but I'm unsure if it exists in the Messenger app. I think I checked once and I could not find it, although it may be there now.
Just a quick tip for those of you who use Facebook messages frequently.
- Franco