Question posted in 
I saw a Facebook marketer use a typical Facebook post where he asked a question like “Do you think X is more effective in marketing or Y? Add your answer in a comment, and I’ll message you the correct answer.”
Upon adding a comment, you immediately received a Facebook messenger message with the right answer, and he used this to start a “conversation” where I’d receive regular updates from him each day until I wrote back the words STOP.
I’ve been scouring the Facebook Messenger API documentation and can’t figure out how you can use this type of interaction on Facebook (not Messenger) as an entry point for a Messenger bot conversation.
I know Facebook has Ads which trigger conversations, but that’s not how this interaction went.
2
Answers
You can use 3rd-party software or build it from Facebook’s graph API. Here’s the only article I’ve found that gives more info (just filter through the bloggy/salesy nonsense and there’s some good info starting about 40% down the page)
https://chatbottutorial.com/fb-post-autoreply/
I know it’s a bit late. This is what I used to reply to a comment.
https://developers.facebook.com/docs/graph-api/reference/v2.12/object/private_replies
BUT you need to do this first: https://developers.facebook.com/docs/graph-api/webhooks/ so that you can subscribe to events where the user comments on a post.
EDIT: As of 2018, you no longer receive “feed” events in your webhook IF your app is in development mode. To solve this, just use the Webhooks dashboard to test and in your webhook endpoint just alter the event with hardcoded data or something.