Messenger's users for a long time feared that Facebook will add ads to their app, and this moment finally came. After six months of beta testing of the ads in Messenger, the company after all decides to add them to the standard version of the app. Update which will advertisements to Messenger will be released during this or next month.
It seems that for now ads will be added only into home screen of app, between our conversations. They probably will take space of three chats. It's unknown if Facebook plans to add more ads to Messenger, but it's nearly sure that in the course of time Messenger will have more advertisements than only one situated in home screen.