Amazon grew into the most powerful online retailer in part by making it effortless for shoppers to buy things with one click. An Amazon effort to become the payment system for other retailers has mostly bombed, though, despite modest improvements in its market position in recent years, new data shows.

The good news for Amazon Pay—as the retailer’s online payments service is known—is that it had six times as many U.S. customers in August of this year as it did at the beginning of 2013, according to estimates by the research firm Second Measure. But while the growth helped Amazon Pay narrow the gap with rival PayPal, PayPal had about 22 times the number of customers using Amazon Pay in August, according to Second Measure. The research firm tracks anonymized purchases and said it is able to identify transactions made through Amazon Pay, which are indicated as such when they are processed. While Amazon Pay has existed in some form since 2007, Amazon launched the service in its current format in 2013.