今更ながらFacebook「Messenger Platform（Beta）」を使ったEchoサーバをRails4で書いてみる ref: http://qiita.com/kskomori/items/d12ba058eb738e3326bf

Facebook_messange_echo_rails4.txt

class FbBotController < ApplicationController

  protect_from_forgery except: :callback

  FB_WEBHOOK_VERIFY_TOKEN="WebHook登録時に入力したトークン"
  FB_PAGE_ACCESS_TOKEN="フェイスブックページのページアクセストークン"

  # WebHooks登録時コールバックURL
  def subscribe
    if params["hub.mode".to_sym] != "subscribe"
      render text: "Error. wrong mode" and return
    end

    if params["hub.verify_token".to_sym] != FB_WEBHOOK_VERIFY_TOKEN
      render text: "Error. wrong verify_token" and return
    end

    render text: params["hub.challenge".to_sym]
  end

  # コールバックURL
  def callback
    messaging = params[:entry][0][:messaging]

    messaging.each do |event|
      # post_text() で送ったものもコールバックされるので判別してる
      # if event.include?(:delivery) でもいいかもしれないがどうだろう
      unless event.include?(:message)
        logger.info("NO MESSAGE(maybe from BOT):sender=#{event[:sender][:id]}")
        next
      end

      from = event[:sender][:id]
      to   = event[:recipient][:id]
      msg  = event[:message][:text]

      post_text(from, msg)
    end

    render text: ''
  end


private

  def post_text(to, text)
    msg = {
      recipient: { id: to },
      message:   { text: text },
    }

    conn = Faraday.new(url: 'https://graph.facebook.com') do |builder|
      builder.request :url_encoded
      builder.response :logger
      builder.adapter Faraday.default_adapter
    end

    endpoint = '/v2.6/me/messages?access_token=' << FB_PAGE_ACCESS_TOKEN
    res = conn.post endpoint do |req|
      req.headers['Content-type'] = 'application/json; charset=UTF-8'

      req.body = msg.to_json
    end
  end

end