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.