Facebook Messenger is an extension of the Facebook usual chat service delivered for years now with no issues whatsoever. The idea was to allow users take their friends with them wherever they go and without the need to have Facebook opened into a web browser.
The result looks like any ordinary chat client, only blue. Pieces of information are provided in the UI about friends’ updates, friend requests and, obviously, chat.
You have a list of your friends and some info on their activity, such as comments, photo tags, photo uploads, likes and everything in any way connected to Facebook.