Facebook Messenger is a chat client that allows user to communicate through text using emoticons and various funny images. Facebook Messenger allows communication between friends or between other users on the same platform. Technically, when you connect to Facebook and chat is enabled, Messenger also connects to the chat server. When you initiate a connection to someone else, the client makes that connection between you-server-client and allows you to send data to the other user. Messages are SSL enabled to avoid MITM attacks.
