On Facebook, it is easy to know whether your message to a particular friend is sent or not. After you have typed your message and clicked on the send button or hit enter. On the right side in the message area, a tick right appears and if the message is delivered then the tick turns blue. But if the message failed to be delivered then you can see a red mark and it informs that "your message is not sent" providing you the option to resend it again or to delete it.