Open the Security app -> Permissions -> auto start apps. Make sure that Facebook is added to the list of apps that may auto start. You should add all apps here who need this to work, like FB Messenger, Whatsapp, Skype, Viber,...
Also in the Permission settings screen, make sure your apps (like FB, whatsapp,...) are flagged as thrusted.
Not mandatory but useful: open the notification bar settings, and allow all those apps to show the notification instead of the three dots.