More than 90 percent of America's online shoppers say they have bought something on Amazon.com.

According to a new NPR/Marist poll, 92 percent of America's online shoppers have shopped Amazon at least once. More than 40 percent of them say they buy something on Amazon once a month or more often.

In fact, when people shop online, they're most likely to start on Amazon. The e-commerce giant's website is the first place shoppers go to when shopping online.

Yet while 92% of customers surveyed use Amazon, nearly three-fourths say it makes up less than 40% of their spending, a figure that seems poised to grow as Amazon ramps up faster shipping and other incentives. 

In the first quarter, Amazon's unique visitors in the U.S. was 183 million--that's 71% of all Internet users. And customer engagement is improving, with a 30% increase in total visits and 26% increase in total minutes customers spent there.

See all the data from the NPR/Marist poll here.