Just days ago, WhatsApp announced it had hit 400 million monthly active users globally — a phenomenal achievement. But in the fickle and trendsetting U.S. app market, Apple’s app store shows just how much WhatsApp’s grip on American consumers has weakened. The period right after Christmas is closely followed by the app industry. These are the days when a flood of new iPhones are being activated, many by teenagers getting their first smartphone. The current trends reflect the priorities of a new generation of smartphone owners. During this peak app purchasing period, WhatsApp faded to No. 63 on the iPhone’s free download chart in the U.S. while Snapchat rocketed to No. 2 and Kik to No. 9. This is perhaps the clearest