After Kelly Clarkson introduced them as the "biggest boy band in the world," the seven BTS members swept across the purple-lit stage in a choreographed routine. BTS fans, who call themselves the A.R.M.Y (Adorable Representative M.C for Youth), went wild, singing along and sometimes breaking into tears. BTS also won the Top Social Artist award for the second year running, beating out superstars like Justin Bieber and Ariana Grande. The award honors artists with the highest fan engagement on social media, recorded and ranked on Billboard's Social 50 chart. As the group accepted the award, they thanked fans around the world in both English and Korean.Popular K-pop groups like BTS have long had devout fan bases in South Korea and across Asia, but only began breaking into the American market recently. BTS made headlines last year as the first K-pop group ever to win a Billboard award, coming in ahead of some of the same big pop names as this year. Since then, the boy band has continued its rise, selling out world tours, performing in Times Square on New Year's Eve and appearing on US talk shows like Ellen. Other K-pop groups like SHINee, Girls' Generation and Exo have enjoyed increasing global recognition, performing at the PyeongChang Winter Olympics and at international K-pop festival KCON. BTS' whirlwind success seems to be growing. The band's new album, "Love Yourself: Tear," sold 1.44 million pre-order copies before its Friday release, and the "Fake Love" music video gained 41 million views on YouTube within 24 hours. Tickets for an autumn world tour have already sold out in 14 locations, and the rest are likely to be snatched up soon.