Tinder, a dating app owned by IAC, has tens of millions of active users and is now worth about $3 billion.

From the day it launched four years ago, cofounder Sean Rad knew he had a hit app on his hands, he says. Downloads rolled in quickly, and engagement metrics were off the charts.

"We texted [Tinder to] literally 500 people," Rad said of the launch. "Immediately, 80% of the people we texted signed up. The next day, we grew 50%. And I thought, 'Wow, that's shocking.' The metrics and the engagement were shocking. We had to fact-check what we were seeing."

Rad detailed the marketing tricks his team used to get Tinder off the ground in an interview for Business Insider's career-focused podcast, "Success! How I Did It."

Picture: (Getty Images)

Here's an excerpt from the podcast:

Alyson Shontell: How did you know Tinder was an instant hit?

Sean Rad: We had the app. Justin was figuring out how to promote it. Then one day he took everyone's phones and spammed all of our address books, including his own. I thought, "OK, this probably isn't going to work, but let's try it."

We texted literally 500 people. Immediately, 80% of the people we texted signed up. The next day, we grew 50%. And I thought, "Wow, that's shocking." The metrics and the engagement were shocking. We had to fact-check what we were seeing.

It really set in when our friends were telling us their stories. One of my friends was telling me about how he never knew this girl who he would see all the time was interested in him, and they started dating. Within a matter of weeks, we were hearing all these stories, and it shook up our friend group.

Shontell: So your friends were matching in ways they probably never would have.

Rad: People who knew of each other but never exposed any interest were now getting connected. So we immediately knew this could have a huge impact on society.

Shontell: Was there anything else you did to get that initial kick?

Rad: Yes. It was a funny story.

Justin's younger brother was throwing a birthday party for his best friend at USC, and he had a bus going from USC to his parents' home. The bus was going back and forth, so a total of about 500 students. Justin called me one day and said, "Let's pay for the bus and call this a Tinder party." I was like, "It's some poor girl's birthday — what do you mean we're going to call it our party?"

So he called the birthday girl and asked, "Can we make this a Tinder party? We'll spend money and make it bigger and better." And she was really excited about the idea.

So we paid for the bus and put a bouncer at the door and told every student that they couldn't walk in unless they downloaded Tinder. You'd literally have to show Tinder on your phone. So about 400 people downloaded Tinder at USC, and I'm sure no one really knew what they downloaded when they walked in.

But then they went home and opened the app and started matching with each other. It really created a phenomenon within USC.

Shontell: It seems like it really caught on on college campuses, with the first being USC. Did you intentionally do the college-by-college thing?

Rad: We realized after that event that it was an effective means for getting the word out. We also realized our harshest critics would be college students. And if we can win our harshest critics, then we can win everyone else.

Immediately after that, every afternoon, the whole team would leave the office, get in a car, and we would drive by every fraternity and sorority in Los Angeles, then San Diego, then Orange County, and every school we could cover.

Every time we would go to sororities and fraternities and talk about Tinder, we would that night see 100 sign-ups. Every single sign-up in the beginning mattered. We were stopping people on the street, and we'd go into coffee shops and talk to each other like, "Oh, have you heard of that app Tinder? It's such a cool app!" Anything we could do to get the word out we were doing.

I'd take out the app and say "Oh, this is interesting! Who told you about this great app called Tinder?" and yell it in the coffee shop, so people keep hearing "Tinder" in LA. And then what happened — and this was nuts — we sort of cornered the West Coast, which is where we lived. Then in January, everyone went home for break and I guess told their friends.

So in the beginning of January, we had about 20,000 users, and at the end of January, we had 500,000 users — all organic. The growth curve was unimaginable. It was pretty amazing.

For more on how Rad built a $3 billion startup in his 20s, listen to the full episode of "Success! How I Did It", or subscribe on iTunes.