code: @RequestMapping(value="/{subscription}", method=GET, params="hub.mode=subscribe")
  	public @ResponseBody String verifySubscription(
  			@PathVariable("subscription") String subscription,
  			@RequestParam("hub.challenge") String challenge,
  			@RequestParam("hub.verify_token") String verifyToken) {
  		logger.debug("Received subscription verification request for '" + subscription + "'.");
  		return tokens.containsKey(subscription) && tokens.get(subscription).equals(verifyToken) ? challenge : "";
  	}
docs: Handles subscription verification callback from Facebook.
  @param subscription The subscription name.
  @param challenge A challenge that Facebook expects to be returned.
  @param verifyToken A verification token that must match with the subscription's token given when the controller was created.
  @return The challenge if the verification token matches; blank string otherwise.