On Sunday afternoon, Eleanor Dolan celebrated her 17th birthday in Minnesota with 20 of her closest friends. They listened to pop music and traded jokes. When the group broke out into “Happy Birthday to You,” Eleanor pulled a slice of cookie cheesecake close in front of her and pretended to blow out the toothpick she had substituted for a candle on top.

Then, she blew lightly on her computer screen. Miles away, her friends extinguished candles atop baked goods in front of them. The party was taking place over Zoom, a video calling app. Eleanor’s father briefly popped into her screen to take a photo.

Teenagers have jokingly referred to themselves as “Zoomers” online for years; now the name is literal. Overnight, Zoom has become a primary social platform for millions of people, a lot of them high school and college students, as those institutions move to online learning.

Zoom Video Communications is an videoconferencing company in San Jose, Calif., that has been thrust into the spotlight over the past week. On Monday morning, its iOS app became the top free download in Apple’s App Store.

On Sunday, nearly 600,000 people downloaded the app, its biggest day ever, according to Apptopia, which tracks mobile apps. While the stock market crashes, Zoom shares have soared this year, valuing the company at $29 billion — more than airlines like Delta, American Airlines or United Airlines.

Zoom has been preparing for this moment since the new coronavirus began spreading in China in January. Even then it was easy to see that Zoom’s primary customer base — videoconferencing desk workers — would become more reliant on its services while quarantined at home. So the company began closely monitoring its capacity and started hosting free training sessions. In China, Zoom dropped its 40-minute limit for free calls.

But no amount of planning could have anticipated the company’s emergence as a cultural phenomenon used to host parties, concerts, church services and art shows. Zoom could not have prepared to become a meme.

A Facebook group for young people trapped at home called Zoom Memes for Self Quaranteens, founded less than a week ago, has already grown to more than 150,000 members.

College students across the country are going on Zoom blind dates. Parents of sixth-graders at Rosenbaum Yeshiva Of North Jersey organized a Zoom “recess” for their children. Ethel’s Club, a wellness platform, is conducting Zoom tarot card readings, breath work and cannabis hangouts.

It is a high-stakes moment for Zoom, which was founded in 2011 by Eric Yuan, a former Cisco Systems executive. Its sudden cultural cachet also brings new concerns over privacy, security, content moderation, safety for young people and sensitivity to the seriousness of the pandemic. There’s also the tiny matter of keeping the service up and running.

“This is a very critical moment,” Mr. Yuan, Zoom’s chief executive, said on an analyst call earlier this month. “Overnight almost everybody read and understood they needed a tool like this.”

‘We finally figured out what the Z stands for in Gen Z’

Virtual gatherings are proliferating. Harvard University, like many schools, has canceled all in-person graduate and undergraduate classes and will conduct them via Zoom.

A common joke among college students is that they all go to “Zoom University” now — the same school, just with very different price tags. Zoom University merch is already for sale on Amazon and RedBubble.

“We finally figured out what Z stands for in Gen Z,” a college student in the Zoom meme group joked.

Many students say that adjusting to school closings and public health guidelines to isolate has been incredibly hard. They have used Zoom to attempt to replicate some sense of normalcy. Parties, sorority socials and beer pong nights have found a new home on Zoom.

Some students developed Zoom-themed drinking games for Zoom parties, adjusting the popular game “never have I ever” to “never have I ever left quarantine.”

“If someone can figure out how to invent a party atmosphere in this socially distant format, then I think it will be a mainstay,” said Lucas Moiseyev, a senior at Carnegie Mellon. “Twitch is to YouTube as Zoom can be to TikTok.”

Mr. Moiseyev thinks the platform has potential to become part of Gen Z’s daily life, post-coronavirus, if the platform can incorporate more social features.

Students say they want things like better direct messaging, more funny face filters and the kind of stickers, live video effects and editing tools they get on platforms like TikTok and Snapchat.

They also want to be able to discover fellow Zoomer hangouts. (The only way to find a Zoom party right now is through a link to it from elsewhere.)

For the time being, young people are being creative with what they have. Teenagers have created TikToks of Zoom hacks, like how to make it appear as if you’re in class when you’re not.

Uploading custom backgrounds has become a way for multiple users to screen share funny memes or videos. At a Zoom party on Saturday, one college student figured out how to adapt a meme and loop a video of Ricardo Milos, a male stripper, behind him.

As new Zoom users flock to the platform, social norms are still evolving. Michael Crisp, a student at Kansas State University, tweeted: “i’m unfamiliar with zoom etiquette. do we gotta ask to leave to go to the bathroom or what. can i have food? can my cat ride shotgun? do i absolutely need pants? this is my HOME bro.”

Any Zoom event with too many callers can also become chaotic. — Some people leave their microphones on, chat nonstop in the sidebar, flip their backgrounds around.

On Saturday, a midnight Zoom party for young climate activists hosted by Ayisha Siddiqa, 21, a founder of Polluters Out, attracted more than 80 attendees, mostly teenagers. After about 30 minutes, the party got so big and boisterous that everyone broke out into smaller rooms.

‘Out of nowhere, Zoom has clout’

But why Zoom? Why not Skype, which has been around since 2003? Or Google Hangouts or Facebook Messenger or Apple’s FaceTime? They’ve all had a boost recently.

Marco Polo, a video chat app, saw sign-ups increase nearly threefold last week over the previous week, the company said.

Zoom is baked into many colleges and schools already that use it. The product’s layout makes it easy to talk with multiple people at once. And Zoom has some features that mirror social media apps. A button called Touch Up My Appearance casts a soft focus over the video display, smoothing out the skin tone of the presenter like an Instagram filter. Custom backdrops can hide messy bedrooms.

Zoom has a “hotter brand” association,said Rishi Jaluria, a senior research analyst at D.A. Davidson. “Younger people don’t want to use the older technology.”

Joshua Rush, 18, a high school senior in Los Angeles, said: “Out of nowhere, I feel like Zoom has clout.”

People also pick Zoom because it works. Paul Condra, a technology analyst at PitchBook, said Zoom’s reliability and simplicity has made it the “standard” in videoconferencing software.

“This is a crisis tailor-made for Zoom,” Mr. Condra said.

As with all products, users should be careful embracing Zoom without being aware of some of the privacy issues. Jules Polonetsky, the chief executive of the Future of Privacy Forum, warned that Zoom’s terms of service includes some stipulations that could overreach into invading user privacy.

“The standard Zoom privacy policy allows data to be shared for targeted advertising,” Mr. Polonetsky wrote in an email interview. And some of the company’s standard terms are not consistent with the Family Educational Rights and Privacy Act, or FERPA, “in addition to many of the 130+ state student privacy laws passed since 2014,” he added.

For workers using the software during business hours, Zoom also includes a feature that can track some aspects of whether a participant is multitasking on a computer and report it back to the host of the call.

“Most users will have no idea,” Mr. Polonetsky said. “Zoom should make it very obvious when this setting is enabled or some people are going to be extremely embarrassed.”

A Zoom spokeswoman said this feature is designed for employers to ensure workers complete training. It is switched off by default.

There is also concern that as Zoom grows in popularity, the company will run into some of the content moderation issues familiar to companies like Facebook, Twitter and other social networks, where people have posted suicides, traded in child exploitation material and have used the platforms to live-stream mass shootings.

Last year a court case in Pennsylvania showed that Zoom was used to distribute child pornography.

“With much broader adoption, abuse and misuse will follow, so Zoom should be getting ready to handle reports and complaints,” Mr. Polonetsky said. But because Zoom is set up as a conference call service, not a publication platform like Facebook, it may face fewer issues than the traditional social networks.

A Zoom spokeswoman said, “Zoom is designed to be used as a business service, and our user policies explicitly prohibit any illegal or abusive activity or content on the platform, including any activities that threaten, exploit or otherwise harm children.”

Here for the duration

On Monday morning, teachers at thousands of schools across the United States dialed into Zoom for the first time and began delivering virtual lessons. Zoom executives, including Mr. Yuan, did triage — on Zoom, of course — from their homes in Silicon Valley.

The schools were taking advantage of Zoom’s decision, announced last week, to make its services free for kindergarten through high schools in the United States, Italy and Japan.

Zoom operates a “freemium” business model: Groups of up to 100 people can use it for 40 minutes at a time at no charge, but must pay $14.99 per month or more for extra features, like bigger groups and administrative controls.

It’s not clear whether the influx of families, teenagers and tarot card readers will translate to an influx in revenue for Zoom. Mr. Jaluria, the D.A. Davidson analyst, said that over time, the company “will get a benefit from this massive brand building that’s happening,” especially when the college students using it today enter, or hope to enter, the work force in a few years.

Zoom has carefully avoided coming off like a tone-deaf Silicon Valley company taking advantage of a crisis. (But it was busy. Citing the work of onboarding schools to the product, leadership was unavailable to talk to The New York Times.)

The epidemic has broadened Mr. Yuan’s view of what it means to be a video communications provider, he wrote in a blog post last month. “I’m compelled to help anyone who needs it,” he said.

On Saturday night, Claire Tran, 22, hosted her first Zoom party after being holed up in her Washington, D.C., apartment for nearly a week. Twenty-one of her friends popped in throughout the four-hour event.

“Before we started the call I was like, this is cool, maybe we’ll do this once a month,” she said. “After it ended, I was like, I need this every weekend or I’ll go crazy.”