Generally, if a company can claim more than half of any age demographic among its customers, it would consider that a major success story. For Facebook (NASDAQ:FB), however, being used regularly by 51% of Americans ages 13 to 17 represents a sharp decline in popularity.

According to Pew Research's 2018 Teens, Social Media & Technology report, Facebook has fallen from the top spot among social media platforms to fourth place. Most alarmingly, since the last time Pew conducted a similar survey in 2015, its usage among teens declined from 71% to 51%, even though the percentage who say they're online "almost constantly" has risen from 24% to 45%.

Facebook now lags behind YouTube (85%), Instagram (72%), and Snapchat (69%) when it comes to digital services used by the 13- to 17-year-old cohort, according to the survey of 1,058 parents of teens and 743 teenagers. Of course, while the declining popularity of the platform among young people is significant for the company, the pain is somewhat mitigated by the fact that the social media giant owns Instagram.

A billboard of the Facebook thumbs up symbol

Fewer teens are giving Facebook a thumbs up. Image source: Facebook.

Smartphones are nearly ubiquitous

Nearly every teenager surveyed (95%) either has a smartphone or has access to one. That's an increase from 73% who owned or had access to a smartphone in 2015. In theory, more teens with mobile access should lead to potential audience growth for all digital platforms.

In reality, while Instagram's audience in the demographic grew from 52% to 72%, lesser player Twitter retained roughly the same share (32% versus 33% in 2015) while Tumblr (acquired in the interim by Verizon) saw its audience drop from 14% of teens to 9%. YouTube, which is owned by Alphabet, was not included as a choice in the 2015 report -- but another Alphabet property, Google+ was. It was then used by 33% of teens, but in 2018, it has fallen off the report entirely

"For the most part, teens tend to use similar platforms regardless of their demographic characteristics, but there are exceptions," wrote Pew's Monica Anderson and Jingjing Jiang. "Notably, lower-income teens are more likely to gravitate toward Facebook than those from higher-income households -- a trend consistent with previous Center surveys."

What, Facebook worry?

On one hand, Facebook should be concerned that it is losing relevance among teenagers. Conversely, the company deserves credit for essentially buying its own hipper, younger replacement with its 2012 purchase of Instagram.

It's hard to know how much these numbers should trouble Facebook. If teenagers use the platform in smaller numbers, but adopt it later, then the company will have nothing to worry about. That scenario seems likely because the social media leader may not be as hip as it once was, but it's still the place where the older demographics congregate.

To teenagers, Facebook has become the social media platform of their parents. It's easy to see why that would push them to Instagram and Snapchat, but as they become 20- and 30-somethings looking for ways to keep in touch with family as well as friends, their digital behaviors may change.

If they don't, Facebook's overall user base will eventually start to shrink -- but even that may not be a real problem as long as those users keep gravitating instead to Instagram.

Suzanne Frey, an executive at Alphabet, is a member of The Motley Fool's board of directors. Daniel B. Kline owns shares of Facebook. The Motley Fool owns shares of and recommends Alphabet (A shares), Alphabet (C shares), Facebook, and Twitter. The Motley Fool owns shares of Verizon Communications. The Motley Fool has a disclosure policy.