Palo Alto, Calif. -- The shopping app Shopkick on Wednesday announced that Nielsen ranked Shopkick No. 34 out of approximately 1 million apps in December 2012. While still trailing giants like Facebook, YouTube and Twitter overall, Shopkick dominated the shopping category during the holiday season, with more frequent and prolonged usage than all physical retailers' own apps including Starbucks, Walmart, Walgreens, Kroger and Safeway, the app developer noted.