Ma Jiajia was too busy setting up a sex toys store, Powerful, to attend her college graduation ceremony in Beijing in 2012.

Clip_8Now Ms. Ma is unveiling a social contact app for customers to meet and share tips about her store’s products. At just 22, she is part of China’s trend-setting, post-1990 generation that is giving shape to enormous social and cultural changes: Sex, she says, should be openly talked about, “fun” and “funny,” not “dirty” or “depressing” and relegated to the far corners of people’s minds. Her app, with its bright colors and playful icons, is a symbol of that shift.

What happens after people meet is up to them, Ms. Ma said.

“The technology puts people in touch. What happens later isn’t a question of technology,” she said in an interview in her black-and-pink decorated store in the city’s Gaobeidian district, balancing on her knees a computer with a prototype of the app’s yellow interface, the color a play on a Chinese word for pornography.

Ms. Ma is alluding to an open secret: Millions of young Chinese are plunging enthusiastically into the world of social contact apps, often location-based and with a hookup element, a dozen people said in interviews.

“On the one hand there’s the new technology, and on the other hand there’s sexuality and desire, and these two are coming together and producing new possibilities,” said Song Shaopeng, a scholar of Marxism and history at Renmin University of China in Beijing.

While WeChat is an established market leader, Momo is rising fast with about 50 million users by July, said the company’s spokeswoman, Wen Yajuan. “Mo” is part of a Chinese word meaning “unfamiliar” or “stranger.”

Lin Yang, 24, said he hooked up last year with a woman he met on Momo. They had sex but “not at home,” he said, declining to be more specific.

Assignations often take place in hotels rented by the hour, several people said.

Mr. Lin, who works at a tech start-up in Beijing, said he initially questioned why a woman would get involved in casual sex. “I don’t really understand how girls think,” he said.

“Then I thought, if I am doing it, why shouldn’t they?” he said by telephone. “I think they’re exploring their sexual freedom. She was so open and free.” He is currently seeing another woman he met on Momo.

Xiao Bao, 24, a graduate student in gender studies, who asked to be identified only by her nickname, has hooked up about four or five times since she broke up with her last boyfriend. Sex was in a hotel, she said, adding, “You don’t always take it all the way.”

“But it’s exciting,” she said. “There’s no love. It’s about sex.”

The app hookup scene, with its high degree of secrecy and anonymity, offers opportunities to explore different kinds of sexuality.

Currently Xiao Bao is dating Sun Sun, a female medical student who used to date men. (She also asked that only her nickname be used.) Sun Sun has tried to hook up via an app called The L, but “I haven’t succeeded yet,” she laughed.

Intrigued, I joined Momo. A friend took a photo of her pewter-furred, caramel-eyed cat. We registered me as Fifi, with the cat as my image. Several nearby men quickly offered a “Ni hao” — Chinese for “Hello.” One, who called himself Dezz, was direct — and persistent. He wrote in English, not uncommon among the technically savvy. I told him I was a reporter. That didn’t deter him.

“Wanna try it out?” he wrote.

I demurred, explaining I was married. Not a problem. Plenty of women using Momo, he wrote, are in relationships, but “they feel boring that’s why they looking for a new kick.”

At Momo’s offices in a glossy Beijing office tower, Wang Li, the wispy-bearded chief operating officer, was wearing a T-shirt, jeans and flip-flops. His business card gave his English name as Wanderlust.

Mr. Wang, who is from a small town in Shanxi Province, says he is aware that hooking up is part of the app’s attraction, but believes there’s more to its popularity — the serious issue of growing loneliness and anomie amid fast-paced urbanization.

Growing up, he and his family knew all their neighbors, but all that has changed. “Over these past two years particularly, social changes have been so incredibly fast as people move to cities and towns,” he said. “Chinese people are fairly introverted,” he said, and in a new environment, “They tend not to make friends with their neighbors.”

“I’ve myself lived in many districts in Beijing and never got to know any neighbors,” he said. “With this technology I can contact them.”

Then there’s how the app can help people who may have married the wrong person find new life partners, Mr. Wang said.

“Chinese people didn’t have choices before and people are getting to middle age and realizing they don’t actually get on with their spouse, because they got married for reasons other than love,” he said. “The younger generation has more choices, and I think that’s a good thing.”

Mr. Wang said the app offers three types of contact: individuals making friends with others, forming groups like sport or restaurant clubs (Ms. Wen, the Momo spokeswoman, said there are 700,000 registered groups) and locating resources like bars or shops.

The company patrols the app to maintain moral standards, Mr. Wang said. Log on and you will be warned that administrators will delete pornography or other inappropriate content, including soliciting for sex.

“If both parties are willing to meet, we don’t interfere,” he said. “But the situation is that things aren’t always that polite or respectful and we don’t want that in our app, so we expel the ‘ugly customers.’

“It’s like at a party,” he said. “If someone drops his pants, well, we’ll kick him out. And we are strict.”