Facebook will only allow your bot to message someone that has already messaged you first.  This would prevent you from initiating a conversation with someone who has never messaged you before.  However, if the person had already messaged you and you were able to link them to their "post" then it should be possible to send them a message.  However, the most common solution is to respond to the post and ask the user to directly message you, that way they can initiate the conversation via messenger.