Since its inception back in September, Samsung Pay managed to reach 5 million users and $500 million in transactions, which is a pretty remarkable feat, considering that it's only available on a few phones. For comparison, Google's Android Pay also has 5 million users, while Apple Pay has 12 million, but it has been available for much longer, making Samsung Pay the fastest-growing proprietary mobile payments platform.
The quick adoption of Samsung Pay is undoubtedly due to the buyout of LoopPay. Samsung acquired a company that allows you to do mobile payments the way you do with existing magnetic stripe cards, and merchants don't even have to retool their systems, like they have to do for Apple's system.
Apple Pay requires merchants to equip their POS terminals with dedicated equipment, that is why the uptake has been somewhat slow so far, but Samsung's decision to directly use LoopPay might quickly make it the most widely used mobile payment system, if its growth trend continues unabated.