SAN FRANCISCO — For teens, it has been an essential rite of passage: They turn 13 and join Facebook.

Since she signed up three years ago, friend requests and status updates are as much a part of Meera Kumar’s life as homework and exams at Menlo School, the elite private school in leafy Atherton, Calif., where she’s a 16-year-old sophomore.

But when her kid sister Anika turned 13 last year, she gave Facebook a pass.

“I guess I haven’t been that interested in it,” said Anika, who prefers sharing photos with friends on Instagram via her iPhone or video chatting with them onGoogle+.

Could Facebook be losing its cool?

With more than 900 million users, Facebook remains the most popular online hangout. But some young people are turning their attention elsewhere. They are checking out new mobile apps, hanging out on Tumblr and Twitter, and sending plain-old text messages from their phones. Their goal is to hook up with smaller circles of friends and share their thoughts and feelings away from the prying eyes of Mom and Dad.

It’s a very grown-up challenge for Facebook, which needs kids to continue to dominate social networking.

Growth in new U.S. users is slowing, and so far, foreign markets, where nearly 80% of Facebook users reside, have proved less profitable. The Menlo Park, Calif., company is reeling from its shaky start as a public company. The botched IPO has heightened concerns about its business, particularly its undeveloped mobile strategy, as teen use of smartphones and tablets explodes. Fickle young consumers can make and break social networks, as evidenced by pioneers such as Myspace and Friendster whose appeal faded as tastes changed.

Researchers who track the technology habits of teens say there is no statistical evidence that Facebook is becoming a teenage wasteland.

“Just because teens are using other services like Twitter and Tumblr more — and they are using these services in huge amounts — doesn’t mean they’re using Facebook less,” said Alice Marwick, a postdoctoral researcher at Microsoft Research, where she studies how teens interact with technology.

In fact, 8 of 10 teens who are online use social networking sites — and more than 93% of those users have a Facebook account, according to the Pew Internet & American Life Project. Millions more kids under the legal Facebook age of 13 fib about their age to use the service.

Still, older people are the ones driving much of Facebook’s growth. Users age 50 to 64 made up nearly a quarter of Facebook’s audience in March, according to research firm Nielsen. The only social network with a higher percentage of older users was professional networking service LinkedIn.

Facebook itself is no longer an adolescent. At 8, it’s getting long in the tooth for a social network. And for some teens, the novelty has worn off.

“Facebook is just not the big fad anymore,” said Kim Franklin, a 15-year-old from Gaithersburg, Md., who does not have a Facebook account and prefers social media site Tumblr. “It was like everybody was constantly on there, but now not so much.”

Franklin said her 13-year-old sister Nicole hasn’t signed up for a Facebook account either.

Meanwhile, Laura Franklin, the girls’ 37-year-old mother, always has Facebook open on her computer while working on her parenting blog, Better in Bulk. That, she said, has led her teen daughters to dub Facebook a “mom thing.”

Turns out the kids are right. A recent study from Nielsen found that nearly 3 of 4 U.S. mothers who went online from a home computer visited Facebook in March. Only 8.3% visited Tumblr, and 14% visited Twitter.

Stats like that make it more likely that younger users will eventually abandon Facebook, said L.A. writer Erika Brooks Adickman. Now 30, she started MyParentsJoinedFacebook.com with fellow twentysomething Jeanne Leitenberg in 2009 to give kids a place to vent when their parents invaded their virtual space.

“When Facebook got started, Paris Hilton was wearing a velour tracksuit,” Adickman said. “At some point Facebook will become the velour tracksuit. There is just a constant evolution, whether it’s what’s going on in fashion or what’s going on the Internet.”

Teens embracing new services say they’d rather use aliases than their real names. They’re just not that into the idea of having everyone see all of their random thoughts. And they also worry that one wrong move on Facebook could hurt their chances of getting into college or landing a job.

On Twitter or Tumblr, they say, they can also be more selective about what they share and with whom, and feel less social pressure to “friend” everyone in their school or friends of friends.

Twitter, they say, just feels more private and intimate. They can use pseudonyms or private locked accounts so their tweets stay between friends. Twitter also enables teens to have fun as a group, jumping on trending topics such as the #IgoToASchoolWhere hashtag.

Tumblr is a creative outlet where they can anonymously express themselves by posting photos, videos or bits of text on topics that interest them, such as Harry Potter, Ryan Gosling or fashion obsessions such as flatforms. They can connect with people they know and get to know people who share their interests, creating a sense of community without revealing more about themselves than they want.

Megan Peranteau, a senior at Poway High School near San Diego, has deleted her Facebook profile six times, and swears this time it’s for good. She has switched to blogging service Tumblr as an outlet for her opinions, photography and art. Now that she’s no longer constantly checking status updates, she and her friends have something to talk about when they meet in person.

“It’s allowed me to create stronger relationships with people I do hang out with, now that they’re not right in my face all the time,” said Peranteau, 18.

Teens who belong to the first truly mobile generation — their most common form of communication is text messaging — are increasingly gravitating to services made for their smartphones and tablets like mobile social network Path. Photo-sharing apps are also very popular, especially Instagram. Facebook agreed to buy the mobile app maker last month for $1 billion to keep it out of the clutches of Apple and Twitter, people familiar with the deal said. Another new service, Snapchat, enables iPhone users to take and send a picture to a friend that is visible only for up to 10 seconds.

Parents are nervous about teens swapping Facebook for services that can popularize risky behavior such as “sexting.” They also don’t like being unable to peek over their teens’ shoulders.

But, Marwick says, that’s the point.

“And honestly, I believe it’s a good thing,” she said. “Teens need a place to socialize and express themselves without grown-ups staring at them.”

Still, for most teens, Facebook is still tops.

Amanda Lenhart, senior research specialist at the Pew Internet & American Life Project, said Facebook has seeped into every nook and cranny of teen life.

“Facebook has been remarkable in its ability to saturate,” Lenhart said. “The only thing that comes close is the land-line telephone.”

Land-lines? Who answers those anymore?

jessica.guynn@latimes.com

ryan.faughnder@latimes.com