Only people who send the message become your subscribers.
How does this work?
User visits your website and checks the opt-in check-box. When that happens, Facebook gives us a temporary ID, which we can use to send messages within 48 hours.
Tobi sends the message. This can be any message, such as abandoned carts or something else.
The user has to reply in order to become a permanent subscriber. Once he’s replied, you’ve got a new subscriber.
Alternatively, the user has to write a Messenger message himself.