Mobile users will be forced to download a separate app if they want to continue to communicate with their Facebook friends.
Facebook has started to remove the chat feature from its main app - meaning users will have to download a separate standalone app if they want to communicate with friends.
The main Facebook application has always had a messaging tab, but from today users in several European countries - including the UK - are being notified the functionality will be pulled.
If they want to continue to chat to their friends, they will have to download the Facebook Messenger app, which like the main app is free.
A Facebook spokesman said: "Today we are starting to notify people that messages are moving out of the Facebook app and over to the Messenger app.
"To continue sending messages on mobile, people will need to install the Messenger app."
Facebook founder Mark Zuckerberg warned of the change in a public question and answer session last November.
In it, he said: "The other thing that we're doing with Messenger is making it so once you have the standalone Messenger app, we are actually taking messaging out of the main Facebook app.
"And the reason why we're doing that is we found that having it as a second-class thing inside the Facebook app makes it so there's more friction to replying to messages, so we would rather have people using a more focused experience for that."
Messaging is one of the big battlegrounds for technology firms, with apps such as Kik, KakaoTalk and WeChat picking up millions of users.
Facebook recently bought free messaging app WhatsApp for $19bn (£11.3bn).
The Facebook Messenger app has a 3.5 star user rating on the Apple App Store, compared to the main app's 2.5 rating.