After trying to setup the Facebook Messenger component without success (because I can’t subscribe as Facebook developer), I tried the method below what is super simple and works like a charm. The setup is not taking more than 3 minutes and it just works.
https://www.callmebot.com/blog/facebook-messenger-free-api-home-assistant/
The messages are received instantaneously! And you can send images 
Sidenote: In the webpage says that it can also be used to send “commands” from Messenger to Home Assistant. I contacted the developer to get more details about this capability.