code: protected String addExchangeToken(final String url, final OAuth2AccessToken accessToken) {
          final FacebookProfileDefinition profileDefinition = (FacebookProfileDefinition) configuration.getProfileDefinition();
          final FacebookConfiguration facebookConfiguration = (FacebookConfiguration) configuration;
          String computedUrl = url;
          if (facebookConfiguration.isUseAppsecretProof()) {
              computedUrl = profileDefinition.computeAppSecretProof(computedUrl, accessToken, facebookConfiguration);
          }
          return CommonHelper.addParameter(computedUrl, EXCHANGE_TOKEN_PARAMETER, accessToken.getAccessToken());
      }
docs: Adds the token to the URL in question. If we require appsecret_proof, then this method
  will also add the appsecret_proof parameter to the URL, as Facebook expects.
  
  @param url the URL to modify
  @param accessToken the token we're passing back and forth
  @return url with additional parameter(s)