When it comes to apps they're using these days, teens and millennials say Snapchat is king — no surprise there.

But second place? It's not Instagram: It's Amazon.

This is according to a survey — The 2017 Love List Brand Affinity Index, run by Condé Nast and Goldman Sachs — that asked 2,345 U.S. millennial and Gen Z shoppers about their fashion, retail and consumer preferences. The survey skewed towards younger consumers.

One question asked which apps they were using currently that they weren't using a few months ago: Snapchat and Amazon came in first and second. (Other popular apps— Instagram, Twitter and Pinterest — came in third, fourth and fifth respectively.)

"Users are looking for efficiency, speed and convenience, and Amazon hits all those buckets," said Conde Nast chief marketing officer Pam Drucker Mann told CNBC.

Buying online allows Gen Z and millennial buyers to get feedback on the products, something they wanted in their shopping experience.

"They're very much paying attention to ratings and reviews," Mann said. "If you're going to brick and mortar, you don't have the same user endorsement that you do online. It's why we think Amazon is doing so well, because they do check all those boxes, allowing people to shop with those things in mind and allowing them to be smarter."

Other apps popular among teens and younger millennials included fashion and shopping apps Poshmark (7), Wish (12), Mercari (14) and rebate app Ibotta (20) — each notable because they were not pre-set answer options, meaning survey takers filled in those names.

"They really pride themselves on being this smart shopper," Mann said. "People had assumed users are really looking for discounts. With this millennial and Gen Zer ... there's a sense of pride or sense of savviness when it comes to shopping, and it's easier to navigate that online."