Hello and Welcome @umaroa to the forum!
I have personally not implemented using Facebook, but these are the resources that will help you to send messages or email to any user information you have collected or stored. You can write the custom message to fetch all stored user information from database and send them bulk messages.
Link 1: To send the message to mobile number: SENDING SMS TO ANY NUMBER WITH RASA - YouTube
Link 2: To send the message to email id: SENDING EMAIL TO ANYBODY WITH RASA - YouTube
I never experimented with numbers to people we can send but you can try.
If you have the user details such as mobile or email id who interacted with the chatbot, then you can easily send the mass messages.
Note: Above links are based on Rasa 2.x
I hope this will help you with your use case. Good Luck!