It's no surprise that Facebook (META -2.70%) and Alphabet's (GOOG -0.28%) (GOOGL -0.27%) Google make the most popular apps in the United States. Both have billions of users around the world for multiple services. Indeed, a recent report from ComScore found that 81% of users had the Facebook app installed on their smartphone and 71% installed YouTube. The two tech giants dominated the top-10 most popular apps.

But when ComScore asked which apps smartphone owners "can't go without," an interesting company shot to the top of the list, particularly in the 18- to 34-year-old demographic.

More millennials named Amazon (AMZN -0.29%) as one of the three apps they couldn't live without than any other app. The app also shot to No. 4 on the charts of apps on users' home screens (behind Facebook, Gmail, and Google Maps), further indicating strong engagement with the app for those that have installed it.

Man holding smartphone.

Image source: Getty Images

Essential even though it's not a time suck

People can get lost for hours in YouTube and Facebook apps. ComScore found 20% of time on mobile is spent using social networking apps. Another 10% is spent consuming multimedia like streaming video. That's great for companies like Facebook and Google, which rely on ad sales for the bulk of their revenue. The more time users spend in their apps, the more ads they can show.

While Amazon's app is indispensable for 35% of millennials, the average user spends just 3% of their time using retail apps. With an average user spending 2.3 hours per day using apps on their phone, that translates into less than 5 minutes per day across all retail apps, not just Amazon, for the average user. For reference, Facebook claims its users spend 50 minutes per day across Facebook, Instagram, and Messenger. Young Instagram users -- the same ones that say Amazon is a must -- spend over 30 minutes per day in the app. YouTube users spend an average of 1 hour per day on mobile alone.

But that's kind of the point of Amazon's app. While YouTube and Facebook want to suck you in and keep you watching videos or scrolling through your news feed, Amazon wants to help you quickly find the item you're looking for and take your money for it.

There's no alternative for Amazon

The thing that makes Amazon indispensable is that it has no replacement. An app like Google Maps is extremely useful on a smartphone, but there's also Apple Maps, Waze (another Google product), MapQuest, and several other navigation apps.

As a shopping experience, the Amazon app doesn't offer much over its own desktop site or other shopping apps. But people love shopping on Amazon. It's often used as a pricing benchmark for people that are shopping in brick-and-mortar stores. It already has shoppers payment information loaded in the app, and it makes checking out quick and painless.

Amazon is becoming Google for products

55% of online product search queries begin on Amazon, and that number is getting bigger every year. That's to the detriment of Google, which can make a lot of money on product-related searches through advertising. Amazon's product search engine makes it as essential (if not moreso) as Google Search on a smartphone.

The majority of e-commerce sales still come from desktop. During last year's Thanksgiving weekend shopping holiday 61% of sales came from desktops despite a majority of e-commerce visitors coming from mobile. But as more people shop from their mobile devices, Amazon is poised to dominate sales growth thanks to the popularity of its app.