CHICAGO — Teens don’t tweet, will never tweet — too public, too many older users. Not cool.

That’s been the prediction for a while now, born of numbers showing that fewer than one in 10 teens were using Twitter early on.

But then their parents, grandparents, neighbors, parents’ friends and anyone in-between started friending them on Facebook, the social networking site of choice for many — and a curious thing began to happen.

Suddenly, their space wasn’t just theirs anymore. So more young people have started shifting to Twitter, almost hiding in plain sight.

“I love twitter, it’s the only thing I have to myself … cause my parents don’t have one,” Britteny Praznik, a 17-year-old who lives outside Milwaukee, gleefully tweeted recently.

While she still has a Facebook account, she joined Twitter last summer, after more people at her high school did the same. “It just sort of caught on,” she says.

Teens tout the ease of use and the ability to send the equivalent of a text message to a circle of friends, often a smaller one than they have on crowded Facebook accounts. They can have multiple accounts and don’t have to use their real names. They also can follow their favorite celebrities and, for those interested in doing so, use Twitter as a soapbox.

The growing popularity teens report fits with findings from the Pew Internet & American Life Project, a nonprofit organization that monitors people’s tech-based habits. The migration has been slow, but steady. A Pew survey last July found that 16 percent of young people, ages 12 to 17, said they used Twitter. Two years earlier, that percentage was just 8 percent.

“That doubling is definitely a significant increase,” says Mary Madden, a senior research specialist at Pew. And she suspects it’s even higher now.

Meanwhile, a Pew survey found that nearly one in five 18- to 29-year-olds have taken a liking to the micro-blogging service, which allows them to tweet, or post, their thoughts 140 characters at a time.

Early on, Twitter had a reputation that many didn’t think fit the online habits of teens — well over half of whom were already using Facebook or other social networking services in 2006, when Twitter launched.

“The first group to colonize Twitter were people in the technology industry — consummate self-promoters,” says Alice Marwick, a post-doctoral researcher atMicrosoft Research, who tracks young people’s online habits.

(Msnbc.com is a joint venture of Microsoft and NBC Universal.)

For teens, self-promotion isn’t usually the goal. At least until they go to college and start thinking about careers, social networking is, well, … social.

But as Twitter has grown, so have the ways people, and communities, use it.

For one, though some don’t realize it, tweets don’t have to be public. A lot of teens like using locked, private accounts. And whether they lock them or not, many also use pseudonyms, so that only their friends know who they are.

“Facebook is like shouting into a crowd. Twitter is like speaking into a room” — that’s what one teen said when he was participating in a focus group at Microsoft Research, Marwick says.

Other teens have told Pew researchers that they feel “social pressure,” to friend people on Facebook — “for instance, friending everyone in your school or that friend of a friend you met at a football game,” Pew researcher Madden says.

Twitter’s more fluid and anonymous setup, teens say, gives them more freedom to avoid friends of friends of friends — not that they’re saying anything particularly earth-shattering. They just don’t want everyone to see it.

Praznik, for instance, tweets anything from complaints and random thoughts to angst and longing.

“i hate snow i hate winter.Moving to California as soon as i can,” one recent post from the Wisconsin teen read.

“Dont add me as a friend for a day just to check up on me and then delete me again and then you wonder why im mad at you.duhhh,” read another.

And one more: “I wish you were mine but you don’t know wht you want. Till you figure out what you want I’m going to do my own thing.”

Different teenagers use Twitter for different reasons.

Some monitor celebrities.

“Twitter is like a backstage pass to a concert,” says Jason Hennessey, CEO of Everspark Interactive, a tech-based marketing agency in Atlanta. “You could send a tweet to Justin Bieber 10 minutes before the concert, and there’s a chance he might tweet you back.”

A few teens use it as a platform to share opinions, keeping their accounts public for all the world to see, as many adults do.

Taylor Smith, a 14-year-old in St. Louis, is one who uses Twitter to monitor the news and to get her own “small points across.” Recently, that has included her dislike for strawberry Pop Tarts and her admiration for a video that features the accomplishments of young female scientists.

She started tweeting 18 months ago after her dad opened his own account. He gave her his blessing, though he watches her account closely.

“Once or twice I used bad language and he never let me hear the end of it,” Smith says. Even so, she appreciates the chance to vent and to be heard and thinks it’s only a matter of time before her friends realize that Twitter is the cool place to be — always an important factor with teens.

They need to “realize it’s time to get in the game,” Smith say, though she notes that some don’t have smart phones or their own laptops — or their parents don’t want them to tweet, feeling they’re too young.

“She could text her friends anyway, without me knowing,” mom says.

Marwick at Microsoft thinks that’s a good call.

“Parents should kind of chill and give them that space,” she says.

Still, teens and parents shouldn’t assume that even locked accounts are completely private, says Ananda Mitra, a professor of communication at Wake Forest University in North Carolina.

Online privacy, he says, is “mythical privacy.”

Certainly, parents are always concerned about online predators — and experts say they should use the same common sense online as they do in the outside world when it comes to dealing with strangers and providing too much personal information.

But there are other privacy issues to consider, Mitra says.

Someone with a public Twitter account might, for instance, retweet a posting made on a friend’s locked account, allowing anyone to see it. It happens all the time.

And on a deeper level, he says those who use Twitter and Facebook — publicly or privately — leave a trail of “digital DNA” that could be mined by universities or employers, law enforcement or advertisers because it is provided voluntarily.

Mitra has coined the term “narb” to describe the narrative bits people reveal about themselves online — age, gender, location and opinions, based on interactions with their friends.

So true privacy, he says, would “literally means withdrawing” from textual communication online or on phones — in essence, using this technology in very limited ways.

But Marwick at Microsoft still thinks private accounts pose little risk when you consider the content of the average teenager’s Twitter account.

“They just want someplace they can express themselves and talk with their friends without everyone watching,” she says.

Much like teens always have.