Katherine was 37, newly single, with family obligations and a full-time job. Tinder does not give out statistics about the number of its users, but the app has grown from being the plaything of a few hundred Los Angeles party kids to a multinational phenomenon in less than a year. I swiped through people I knew from college, people I might’ve recognized from the train.Unlike the robot yentas of yore (Match.com, Ok Cupid, e Harmony), which out-competed one another with claims of compatibility algorithms and secret love formulas, the only promise Tinder makes is to show you the other users in your immediate vicinity. It had taught him that women find me more attractive than I think. It therefore read as mock bravado when Eli wrote, But you ever just want to fuck please please holler at me cool??? I saw it had gone global when a friend in England posted a Tinder-inspired poem on her Facebook page (and here are we, He and Me, our flat-screen selves rendered 3D). The more I used it, the more I considered how much it would have helped me at other times in my life—to make friends in grad school, to meet people after moving to a new city.

He was 27 years old, losing the vestigial greenness of his youth. Around the same time, somewhere across town, a woman named Katherine1 shut down her Ok Cupid account.

He wanted to have sex with some women, and he wanted some stories to tell. She had approached Internet dating assertively, had checked the box that read Short-term dating and the one that read Casual sex.

Then a casual encounter had turned menacing, and Katherine decided she no longer wanted to pursue sex with total strangers. Katherine wrote: You can’t be psycho or I will tell [name of mutual friend]. I first signed up for Tinder in May but found it skewed too young.

But she had a problem: She liked the adventure, she had the usual human need for other humans, and she needed the convenience of meeting people online. When Katherine and Eli downloaded Tinder in October 2013, they joined millions of Americans interested in trying the fastest-growing mobile dating service in the country. He did not, like one guy, start the conversation with Don’t you want to touch my abs? (I’m 32.) When I looked again in mid-October, everything had changed.

Your matches accrue in a folder, and often that’s the end of the story. The swiping phase is as lulling in its eye-glazing repetition as a casino slot machine, the chatting phase ideal for idle, noncommittal flirting. (His tagline made her laugh.) They had one friend in common, and they both liked Louis C. (The third is Jonathan Badeen, the engineer who built the app.) Rad is the chief ecutive officer; Mateen is chief marketing officer.

In terms of popularity, Tinder is a massive and undeniable success. For Katherine, still wary from her bad encounter, Tinder offered another advantage. They are also best friends, share a resemblance to David Schwimmer, and have been known to show up for work in the same outfit. For people his age, the primary way to interface with the technical world was through a mobile device.

It uses your pre-existing Facebook network and shows which friends, if any, you have in common with the person in the photo. I was staying only a mile from Tinder’s offices in West Hollywood, and within forty-eight hours both founders showed up on my Tinder feed. Rad and Mateen have shared business ideas with each other for years, and every idea begins with a problem.

Until recently, hookup apps were straightforward but sleazy.

Then along came Tinder, the dating-hookup hybrid that made things simpler, sexier, and particularly lady-friendly.