From a recent Piper Jaffray Companies' 31st semi-annual "Taking Stock With Teens" research survey with 6,500 respondents across 46 U.S. states:

Amazon ranked first among websites, with a 41% “mindshare.” Its closest competitors, Nike and Forever 21, both had a 5% share. In a related finding, the survey found that Amazon Prime adoption, which has increased across all income brackets in each of the past five surveys, continues to expand and now exists in 51% of households of the teens in the survey. This survey, along with other previous Piper Jaffray consumer surveys, suggests that there are 57-61 million Prime households in the U.S.