Can't get my Facebook messenger bot to respond with response from my service

Question

I have made an Api.Ai bot and integrated it with both Slack and Facebook Messenger. When I write to it, it answers with the responses set up in Api.Ai for both Slack and Facebook Messenger, but on the fulfillment part, when Api.Ai makes a call to my service, it works fine in Slack, but I get no response from Facebook Messenger.

The format of the message I return from my service:

{
"contextOut": [
    {
        "lifespan": 2,
        "name": "weather",
        "parameters": {
            "city": "Rome"
        }
    }
],
"data": {
    "facebook": {
        "message": {
            "text": "Great success!"
        },
        "recipient": {
            "id": "1454102654663349"
        }
    },
    "slack": {
        "attachments": [
            {
                "color": "#00A399",
                "title": "Hello world!",
                "title_link": "https://www.mywebsite.se"
            }
        ],
        "text": "Horray! Great success! :)"
    }
},
"displayText": "Whatever!!",
"followupEvent": {
    "followupEvent": {
        "data": {
            "parameter": "<parameter_value>"
        },
        "name": "<event_name>"
    }
},
"source": "mywebsite.se",
"speech": "Whatever!?"
}

The Facebook recipient id comes from the request made to my service.

request.result.contexts[0].parameters.Facebook_sender_id

I have verified my webhook under the product settings tab in the Facebook app.
I have subscribed my app to the page using my page access token.
I have checked the following events under webhooks: messages, messaging_postbacks
I'm logged in as the admin user of the app, when trying the bot in Facebook.

I'm out of ideas, there must be something I've missed?

EDIT: I've set up an Azure Function as my webhook for testing purposes.

public static async Task<HttpResponseMessage> Run(HttpRequestMessage req, TraceWriter log)
{
    var request = await req.Content.ReadAsAsync<ApiAiMessage>();

    log.Info($"Incoming: {JsonConvert.SerializeObject(request)}");

    var slack_message = new {
        text = $"Horray! Great success! :)",
        attachments = new[] {
            new {
                    title = "Hello world!",
                    title_link = "https://www.mywebsite.se",
                    color = "#00A399"
                 }
            }
        };

    var facebook_message = new {
            recipient = new {
                id = $"{request.result.contexts[0].parameters.Facebook_sender_id}"
                },
            message = new {
                    text = "Great success!"
                }
            };

    var response = new
    {
        data = new
            {
                facebook = facebook_message,
                slack = slack_message
            },
        speech = "Whatever!?",
        displayText = "Whatever!!",
        contextOut = new[] {
            new {
                    name = "weather",
                    lifespan = 2,
                    parameters = new {
                        city = "Rome"
                    }
                }
            },
        source = "mywebsite.se",
        followupEvent = new {
            followupEvent = new {
                name = "<event_name>",
                data = new {
                    parameter = "<parameter_value>"
                }
            }
        }
    };

    log.Info($"Outgoing: {JsonConvert.SerializeObject(response)}");

    return req.CreateResponse(HttpStatusCode.OK, response, new MediaTypeHeaderValue("application/json"));
}

Answer 1

Where are you actually sending the API.ai response back to Facebook Messenger? This requires some FB specifics like the 'page token' and the fact it works for Slack but not FB makes me believe it's just something simple left out like this. Here's an example in javascript of what the call would look like

function callSendAPI(messageData) {
	request({
		uri: 'https://graph.facebook.com/v2.6/me/messages',
		qs: {
			access_token: config.FB_PAGE_TOKEN
		},
		method: 'POST',
		json: messageData

	}, function (error, response, body) {
		if (!error && response.statusCode == 200) {
			var recipientId = body.recipient_id;
			var messageId = body.message_id;

			if (messageId) {
				console.log("Successfully sent message with id %s to recipient %s",
					messageId, recipientId);
			} else {
				console.log("Successfully called Send API for recipient %s",
					recipientId);
			}
		} else {
			console.error("Failed calling Send API", response.statusCode, response.statusMessage, body.error);
		}
	});
}