Facebook (NASDAQ: FB) is having a problem holding the attention of younger users in the United States.

Snap's (NYSE: SNAP) Snapchat is generally perceived as the main app capturing more time from younger millennials and the Gen Z set. CEO Evan Spiegel said users 25 and younger spent an average of more than 40 minutes per day on his company's app in the second quarter.

But in Pew's most recent survey on social media use, it found that the percentage of people between the ages of 18 and 24 who use Snapchat is actually slightly less than those who use Facebook. The only social app with more users in that demographic was YouTube, a subsidiary of Google, which is an Alphabet (NASDAQ: GOOGL) (NASDAQ: GOOG) company.

In fact, YouTube is the most popular social app overall and across every age demographic.

Image source: Getty Images.

YouTube may be more popular than television among the 18-to-25 cohort

Pew found that 94% of U.S. adults under 25 use YouTube. YouTube is often used as a substitute for television among younger users, who have ditched TV at a much more rapid pace than older viewers. Since 2011, the time Americans ages 18 to 24 spent watching television per week fell from 26.5 hours to 12.7 hours.

Meanwhile, the average YouTube viewer spends over an hour per day streaming video on mobile devices, and that number likely skews higher in younger demographics. On computers, viewership trends higher among younger Americans, with those aged 18 to 34 spent an average of 42 minutes per day on YouTube compared to 23 minutes for the entire adult population, according to data from comScore (NASDAQOTH: SCOR).

Not to mention the increasing amount of time people spend watching YouTube on television screens using streaming video players and game consoles. On Alphabet's fourth-quarter earnings call, Google CEO Sundar Pichai said, "We are seeing [YouTube] now grow well beyond mobile, especially in the living room."

The trend of young people in the U.S. spending more time watching YouTube is notable especially for advertisers who are increasingly looking for new outlets as television viewership and cable subscriptions decline.

YouTube presents one of the best digital substitutes, offering similar content and ad formats but with the benefits of digital targeting. The younger audiences that favor it are also coveted by brand advertisers, which means YouTube is poised to win a significant share of the $70 billion U.S. television ad market. One analyst believes it's already a $15 billion business.

Facebook and Snap are playing catch-up

Despite the large amount of time users spend with its app, Facebook's flagship platform is actually seeing a decline in monthly usage among 18- to 24-year-olds. The company is making several changes to its News Feed, which may also negatively impact use time. Instagram, however, continues to see strong and growing engagement among young adults, who use the app an average of 32 minutes per day.