{
  "error": {
    "message": "An unknown error occurred",
    "type": "FacebookApiException",
    "code": 1,
    "error_subcode": 1929002,
    "fbtrace_id": "F9qp3C16CGP"
  }
}

https://graph.facebook.com/{appid}/subscriptions?access_token={appid}|{appsecret}

{
   "object" : "user",
   "callback_url" : "{callbackurl}",
   "fields" : "notifications",
   "verify_token" : "{verification token}"
}