One of the most problems that Facebook users are facing, is when you are trying to delete all your Chat messages. It's kind of impossible to do that especially when your Account is full of messages, unless you keep deleting one by one until you finish that, you may lose too much of time if you do this. This trick is to add a button "Delete All" in the head of message page, when clicked it will remove all Messages that exists on that page, then scroll to the next page.
How to Delete/Remove All Facebook Messages at Once
1) Go to this Google Chrome Extension
2) .Click Add to Chrome
3) Go to your Facebook's messages page. Facebook.com/messages
4) You'll notice a new button named "Delete All"
5) Every time you click that button: You'll delete all messages on that page, then it will bring a new page.
6) Done! Keep doing that until you finish all messages.