Facebook Messenger allows you to send messages to those users who initiated the conversation first (except sending a message using users' phone number). You can listen to Message Received webhook or Postback Received webhook (if you use Get Started Button) and react appropriately.
To drive the initial engagement, Facebook provides tools to let your business be discovered on Messenger, plugins to install on your sites and the ability to run ads that lead users to your Messenger account. See below all different entry points offered by Facebook.
Phone Number - Send Message
It is possible to send messages to users using phone numbers they've associated with their Messenger accounts. As it is you who initiated the conversation, the user will receive your message as pending and will be asked to either confirm or decline your message (see Using Phone Number for more details).
Updated almost 5 years ago