Facebook, for instance, is the largest social network in the world, with more than 1 billion active monthly users. But it didn't garner significant growth among U.S. Internet users in the past three years, according to a new study from the Pew Research Center.
Some 72% of online U.S. adults use Facebook today. That is up only 5 points from 67% in 2012, Pew noted.
By comparison, Pinterest more than doubled its user base, going from 15% of online U.S. adults in 2012 to 31% today. Similarly, Instagram also showed strong momentum, growing from 13% three years ago to 28% now.
Other major players, including Twitter and LinkedIn, also saw growth but not at such a strong pace.
Pew reported that 23% of online adults use Twitter, a 7-point increase from the 16% who used it in 2012. As for LinkedIn, a quarter of online adults use the site, up from 20% in 2012, the survey noted.
"Interesting but not surprising," said Zeus Kerravala, an analyst with ZK Research. "I have six kids under 23 and none of them use Facebook regularly. I think Facebook is almost considered an older person's social tool now... Much of the growth in the younger population is on Instagram, Vine, etc."
And why don't Kerravala's kids use Facebook
Simple. He thinks it's because he and his wife do, so his kids don't think it's cool and want to keep their online social information out of their parents' watchful eyes.
That's going to make it tough for Facebook to continue to grow out its user base. On top of that, Facebook simply has huge market saturation in the U.S. People who want to use the social network likely already are.
That's one of the reasons Facebook is pushing so hard to increase Internet connectivity in parts of the world where there largely is none. With more people online, there are more people able to use Facebook.
"Facebook may not be growing as fast as it used to but this study shows clearly that Facebook is the hub you have to be on," said Patrick Moorhead, an analyst with Moor Insights & Strategy. "Facebook must expand globally to grow its monthly active users."
However, the Pew study also shows that while younger users are using Instagram and Pinterest, they clearly haven't abandoned Facebook.
According to Pew, 82% of online U.S. adults between the ages of 18 to 29 use Facebook, along with 79% of those between 30 and 49, 64% of those ages 50 to 64 and 48% of those 65 and older.
Moorhead noted that while twenty-somethings may be using Facebook, they're not glued to it. They bounce around between that site and Instagram, Snapchat and Twitter.
"What's interesting is the 65-and-over group," he added. "That segment makes perfect sense to me. With families strewn across America at an unprecedented rate, the over-65 group uses Facebook to communicate with family members."