GEOFF BENNETT: Now to our special coverage of teens in crisis.

As we reported yesterday, a national survey by the CDC is raising alarm.

It shows that nearly 30 percent of teenage girls said they had considered dying by suicide, and three out of five girls said they felt persistently sad or hopeless.

All this comes at a moment when there's growing concern about the impact of social media.

During a Senate hearing today, lawmakers called out social media companies for not doing enough to protect teens.

And school districts and hundreds of families are now pursuing lawsuits against the tech giants, seeking to hold them accountable for rising rates of teenage depression, suicides, cyberbullying, and eating disorders.

Special correspondent Christopher Booker reports from New York as part of our ongoing series Early Warnings: America's Youth Mental Health Crisis.

CHRISTOPHER BOOKER: For Nuala Mullen, it started when she was 10 years old, posting videos like this one to social media.

Two years later, she joined Instagram, the next year, TikTok.

NUALA MULLEN, 18 Years Old: It's just an addiction.

Once you know what it feels like to get likes and validation, you just crave it all of the time.

CHRISTOPHER BOOKER: And over the next five years, she gained thousands of followers documenting her teenage life.

NUALA MULLEN: It's like I knew that I was hurting myself and what I was doing wasn't beneficial to me, but I just needed that validation so badly that I was willing to do anything to get it.

CHRISTOPHER BOOKER: Mullen, who is now 18, says that became especially true at the start of the pandemic.

Then a star field hockey player at her high school in Westchester County, New York, Mullen says she started doing popular workout challenges on TikTok and Instagram while stuck at home.

NUALA MULLEN: I think that's really how I fell down the rabbit hole, because I was noticing, after these two weeks, the changes.

And I was getting comments on TikTok being like, oh, you look so good, whatever.

And I thought to myself, oh, something must be working, you know?

CHRISTOPHER BOOKER: Almost immediately, Mullen says her Instagram and TikTok feeds were flooded with body image content, from workout challenges, to diet tips, to testimonials on how to lose and keep off weight.

Before long, she had developed a new routine, one that continued even after she went back to school.

NUALA MULLEN: Well, I would go to field hockey practice, come home.

I would run for an hour.

I would do weight training.

I would do ab routines.

I would do HIIT workout videos, basically until I was too weak to do anything else.

I was training for hours and hours.

And, throughout the day, I wasn't eating then.

ELIZABETH MULLEN, Mother of Nuala Mullen: I had no idea who she was.

It was like another person took over her body.

CHRISTOPHER BOOKER: Nuala's mom, Elizabeth Mullen, says she and her daughter have always been close, but, as Nuala became obsessed with working out, she struggled to understand what was fueling this new behavior.

ELIZABETH MULLEN: She would talk about a feeling of not being good enough, being lonely at times, not being seen.

I was like, what's happening here?

And then I started to really take a look into what she was seeing on the phone.

CHRISTOPHER BOOKER: What was it like for you as a parent to first try to understand what was happening and then, by extension, try to get control over what was happening?

ELIZABETH MULLEN: At its worst, it's like dropping your boat's anchor in the middle of a hurricane at sea.

Like, it is just impossible, because I'd get on and I would like, well, what's this about?

Why do you have to photograph yourself like that?

And so what ended up happening is, she's a smart girl.

She would just create different accounts.

CHRISTOPHER BOOKER: By the fall of 2021, Nuala's life began to spiral.

Diagnosed with anorexia, she began having chest pains and was hospitalized after her heart rate became dangerously low.

NUALA MULLEN: For me, I couldn't get skinny enough.

I couldn't receive enough likes.

I was just still in that mind-set that I needed to be skinny in order for these people online to like me.

CHRISTOPHER BOOKER: What about peers and friends?

Did you have conversations with them later about what had happened?

NUALA MULLEN: Not until after my second hospitalization.

I found that even, like, during the eating disorder, I was - - I didn't want to tell anyone, not even in the sense that I was embarrassed, but it was competitive for me.

I thought, oh, if I shared that I had anorexia with one of my friends, they might get a notion and they might become skinnier than me and they might get more likes.

So I wouldn't tell anyone what was going on.

CHRISTOPHER BOOKER: In December, the Mullens filed a lawsuit against both TikTok and Meta, the parent company that owns Instagram and Facebook, alleging that the addictive qualities of these platforms are causing and contributing to the burgeoning mental health crisis for teenagers.

It's one of hundreds of lawsuits against social media companies that come as the industry faces increasing calls for reform, including from President Joe Biden earlier this month.

JOE BIDEN, President of the United States: We must finally hold social media companies accountable for experimenting they're doing running children for profit.

(CHEERING AND APPLAUSE) CHRISTOPHER BOOKER: Social media companies have long been shielded from lawsuits because of what's known as Section 230 of the Communications Decency Act, a 1996 law that protected the companies from what users post on their platforms.

But the Supreme Court will consider challenges to the law later this month.

IMRAN AHMED, CEO, Center for Countering Digital Hate: Right now, platforms have no responsibility for how their businesses cause harm.

CHRISTOPHER BOOKER: Imran Ahmed is the CEO of the nonprofit Center for Countering Digital Hate.

In their recent report titled "Deadly By Design," the organization calculated that videos related to eating disorders on TikTok had been viewed more than 13 billion times.

The organization also set up eight TikTok accounts, all posing as 13-year-olds, the minimum age allowed by law to be on social media.

After these accounts briefly viewed or liked body image and mental health content, more was quickly fed to them.

IMRAN AHMED: Within two-and-a-half minutes of opening an account as a 13-year-old girl, it's sending its self-harm content, within eight minutes, eating disorder content.

Every 39 seconds in the first half-hour, they were receiving some sort of harmful content.

CHRISTOPHER BOOKER: Both TikTok and Meta declined the "NewsHour"'s request for an interview, but a TikTok spokesperson told us that, last year, the company proactively removed more than 80 percent of all eating disorder content within 24 hours, and more than 70 percent of those videos received no views.

While Meta told the "NewsHour": "We want teens to be safe online and we don't allow content that promotes suicide, self-harm or eating disorders."

The statement goes on to say: "Of the content we remove, we identify 99 percent of it before it's reported to us."

JONATHAN HAIDT, New York University: If you get users when they're young, they will -- there's a good chance it'll stay on for life.

Everybody's competing for the teenagers.

CHRISTOPHER BOOKER: Jonathan Haidt is a social psychologist at New York University.

He traces what he calls an epidemic of teenage sadness back more than a decade.

JONATHAN HAIDT: So, it's anxiety and depression, also self-harm and suicide.

All of those things skyrocket after 2012.

CHRISTOPHER BOOKER: Just to clarify, this isn't just social media?

There are other factors at play?

JONATHAN HAIDT: Yes, there are always other factors at play.

This is a complicated sociological phenomenon.

But the instant they go on social media -- by instant, I mean, like within a year -- the depression lines begin to go up.

Plus, there is direct correlational evidence that the more you use it, the more depressed you are.

It's especially heavy users, more than four hours a day.

Those girls are three times more likely to be depressed than medium users.

CHRISTOPHER BOOKER: Despite these concerns, teens are more likely to view their own time on social media as positive, rather than negative, saying it makes them feel more connected with friends and offers them a support network.

But more than a third of all teenagers say they're on at least one social media platform almost constantly.

DR. VIVEK MURTHY, U.S.

Surgeon General: Age 13 is when kids are technically allowed to use social media.

CHRISTOPHER BOOKER: That has led to calls from health experts to increase user age requirements to join social media, including the U.S. surgeon general, Dr. Vivek Murthy.

DR. VIVEK MURTHY: I personally, based on the data I have seen, believe that 13 is too early.

CHRISTOPHER BOOKER: In the fall, Instagram rolled out a host of new parental supervision tools, which give parents the ability to monitor how long their children are on the platform, what accounts they follow and who follows them.

They also include reminders for teens to take a break.

TikTok also highlights that, by default, the accounts of users between age 13 to 15 are set to private, and they restrict their direct messaging.

ELIZABETH MULLEN: I don't know where that content is going.

I don't know who is looking at it.

CHRISTOPHER BOOKER: But Elizabeth Mullen believes not enough is being done to protect users like her daughter, who remains on both TikTok and Instagram, and is still seeing much of the same content as before.

ELIZABETH MULLEN: The app is working against us,for sure.

The only difference between now and then is, if she is confronting it and we do see her sliding, there's more people to jump on, whereas, before, we didn't know what was happening.

CHRISTOPHER BOOKER: Nuala was forced to quit her field hockey team last year because of health concerns, but, today, she says, overall, she is in a far better place.

NUALA MULLEN: On social media, everyone posts the highlights of their life.

No one acknowledges what's happening behind the screen.

People probably thought I was the happiest then, but it was one of the worst times of my life.

CHRISTOPHER BOOKER: For you as a young person, what would your life be like if you're not on social media?

Because I think that's one of the questions.

Just get off.

NUALA MULLEN: I think, without social media, that I would be completely left off.

And that's just how things are now.

I mean, I could get off the app, sure, yes, but then I wouldn't be able to talk to my friends.

I wouldn't be able to make plans.

I wouldn't be able to see what people are doing.

And, in this day and age, it's so hard to stay off of it because everyone's life revolves around it.

CHRISTOPHER BOOKER: Nuala is now trying to build a new life.

She recently accepted an academic scholarship to Fairfield University in Connecticut, where she will begin in the fall.

For the "PBS NewsHour," I'm Christopher Booker in New York.