Nicole found out the guy she was dating was already in a committed relationship. Abby learned that her ex had most likely hooked up with someone new, and Ben discovered that a long-ago casual fling had apparently developed a drug habit.

The sleuthing tool that cracked these relationship mysteries was not a private investigator, but the peer-to-peer payment app Venmo.

The mobile payment service, which processed more than $35 billion in payments last year, is a no-fuss solution for splitting the dinner bill after a night out with friends.

But Venmo users have found it’s also an extremely effective tool for keeping tabs on friends, partners and exes, researching crushes, and in some cases, uncovering infidelity. Some even say Venmo is a better method for watching people than more explicitly public social media platforms like Facebook FB, -2.24% or Instagram.

Some users seem to forget that their transactions are public by default, and their payment activity provides an unfiltered paper trail of what’s really happening in their lives.

“What you’re seeing on Instagram or Facebook is what they want you to see,” said Abby Faber, a 19-year-old freshman at Indiana University. “They’re edited pictures that they put up. But with Venmo, it’s very normal casual interactions. It’s what they were doing and spending money on.”

In her case, she checked up on her ex-boyfriend and saw he was spending money on pizza and the popular video game Fortnite—and making regular payments to one girl, who Faber guessed is his new hook-up.

She also did some fact-finding on a new crush and saw that she may have competition: he had recently donated to another girl’s charity event. “Not that I care,” she said. “It’s just interesting to see.”

The social feed is Venmo’s ‘secret sauce’

Venmo has had a social component since it launched in 2009. Users see a feed of both their own friends’ payments and total strangers’ activity every time they open the app, and it’s easy to look up users. Exact amounts aren’t listed, but you can see who’s paying who and which words or emoji they use to describe the payment.

That’s bad news for people who use Venmo to pay their drug dealer and then actually write “drugs” in the payment’s description field, but great for amateur detectives. One Chicago woman told MarketWatch she used to do “minor celebrity stalking” of “Saturday Night Live” cast members and former Disney Channel DIS, -3.08% child actors on the app.

“ ‘She paid on Venmo because she wanted me to know she was hanging out with those people.’ ”

The social feed is Venmo’s “secret sauce,” said Erin Mackey, a spokeswoman for Venmo and its parent company PayPal PYPL, -3.28% . In fact, it’s usually the reason people are logging on. “Our most active users check Venmo daily and the average user checks Venmo two to three times per week—and it’s not for payments, but to see what their friends and family are doing.”

Privacy options have changed

Venmo’s public-by-default social component caught the eye of the Federal Trade Commission in 2017, and the agency accused Venmo of “misleading” users about the fact that they needed to change two separate privacy settings to make their transactions completely private. Venmo reached a settlement with the FTC, and a company spokesman noted that users now have three options for controlling who can see their payments. “Payments are very personal at heart,” said spokesman Pablo Rodriguez. “Just like with anything that’s social, you have to decide how much you want to share.”

The Moneyist: How to deal with a stingy friend

‘Fellas, it ain’t safe out there!’

When it comes to matters of the heart, Venmo users see the app’s social feed as either an asset or a negative, depending on their relationship goals. “Thank you, Venmo, for publicly showing them receipts,” tweeted a grateful woman after the app’s payment history confirmed her suspicions about an ex-boyfriend. “Of all the social media platforms to find out your ex was shady about his last relationship,” she wrote. “Venmo came through like Twitter & IG never did.”

Meanwhile, another user warned would-be cheaters—“Fellas, it ain’t safe out there!”—after his coworker busted her cheating boyfriend with the app.

‘Guys really suck’

Nicole Vavro, a 20-year-old college student in Cincinnati, says she’s glad she learned a tough but valuable lesson after some Venmo detective work. Vavro had been dating a guy for about six months, but couldn’t shake a weird feeling about him.

His Facebook, Instagram and Twitter TWTR, +6.08% were all private—which, in retrospect, should have set off alarm bells, she says now. So Vavro turned to his Venmo feed for insight. She noticed regular payments to one girl, often accompanied by “flirty” descriptions.

Vavro looked up the girl’s Facebook page and bingo: She was in a committed relationship with the guy Vavro was dating. Vavro confronted him, and he admitted that he hadn’t been honest about his relationship status and begged Vavro not to tell the other woman. “It taught me so much,” Vavro said. “Guys really suck.”

Nicole Vavro says she learned a tough but valuable lesson after some Venmo sleuthing.

‘I sat in bed and scrolled’

Ben Ryan, a 28-year-old Jersey Shore resident, came across an old fling on Venmo and couldn’t resist checking to see what he had been up to. “I saw his name come up and I was, like, ‘Oh, this is going to be good,’” Ryan told MarketWatch. “I couldn’t believe I was actually doing this. I was going back to 2016. I sat in bed and scrolled.”

His takeaway: He was a little bit jealous to see some of his friends hanging out with his one-time hook-up. But he decided it was a good thing their relationship didn’t last, because there were a lot of snowflake emojis in his payment history. Users often use the image when they’re buying drugs.

“ ‘The average user checks Venmo two to three times per week—and it’s not for payments, but to see what their friends and family are doing.’ ”

Why Venmo doesn’t feel like other social media

What is it about seeing money change hands on Venmo that feels different than viewing someone’s vacation photos on Instagram? In relationships, money can be fraught, sometimes signifying power, trust, and control, says New York City psychotherapist Matt Lundquist of Tribeca Therapy.

It works both ways, too. He’s seen clients use Venmo as a tool to hurt each other. A client going through a breakup recently told Lundquist, “She paid on Venmo because she wanted me to know she was hanging out with those people.”

When a couple is splitting up, spending money in certain ways can be a declaration of independence, Lundquist said. Exes can use Venmo to say: ‘You always thought it was dumb I spent money on these things, now here I am, doing it.”

And sometimes a payment transaction can feel like an infidelity, even if it isn’t. “Two people are going out, they’re starting to get serious, and they might feel sexually committed, then it gets found out through Venmo that one person in the relationship bought dinner for someone else, and that feels like cheating,” Lundquist said.

There’s a cryptic tease to Venmo

There’s also something tantalizingly cryptic about Venmo. No dollar amounts are listed, and users can use whatever words or images they want to describe each transaction, so there’s an element of mystery. Why did your college friend pay one of your mutual pals “just for being you”? Who knows? It’s transparency with a twist of the unknown.

For some, it’s just enough to feel connected to friends a half a world away.

Aaron Rayburn loves Venmo for keeping up with friends in the U.S. while he’s living in Indonesia. “I recently moved overseas and, therefore, don’t know what my core group of friends are doing on a weekly basis,” Rayburn said. “Seeing their small financial interactions actually gives me a window of who is hanging with who, and it warms my heart. It’s a way to check in, with zero interaction.”