The Breakup Notifier app lets users select the friends whose love lives they want to monitor and, whenever they change their relationship status on Facebook, it sends an e-mail.
"You like someone. They're in a relationship. Be the first to know when they're out of it," says the tagline on the app's website.
Dan Loewenherz, 24, a developer based in Beverly Hills, Calif., said it took him just about four hours to build the application. He launched the app and its website Saturday, and in the past 36 hours the site has been visited more than 700,000 times and the app has been downloaded by 40,000 people, he said.
"I was blown away," he said. "This weekend, I just thought it would be a fun thing to do. It was going to be a little joke, I was going to send it to some friends… but I think people really like this idea. I just didn't intend it to be this big."
The app is currently free, but Loewenherz said that given the tool's popularity, he might charge users for the service. A possible plan would be to let people monitor one or two friends for free and then pay one or two Facebook credits to monitor additional friends.
In a discussion thread on a popular tech forum, Loewenherz said he developed the app after a conversation between his fiancee and her mother.
The pair was talking about setting up his fiancee's sister with a guy who was already attached, when Loewenherz's future mother-in-law joked that it would nice to know when he was single again.
"I blurted out that I could make something that could do that in a couple of hours. By then, I knew I had to do it," he wrote.
Originally, the app scanned Facebook relationship statuses every 24 hours and sent e-mails about the changes. Loewenherz has since updated it to scan for status changes every 10 minutes.
Loewenherz recently quit his job to launch a start-up called Crate, a file-sharing application.
As for the inspiration for the stalking tool -- his mother-in-law to be -- Loewenherz said she's enjoying his latest Web creation.
"She thinks it's hilarious. She's got a ball out of it. She's extremely amused," he said.
In other relationship-related Facebook news, the site last week announced that it was adding civil unions and domestic partnerships to the relationship status options on user profiles.
Previously, users could describe themselves as single, married, in an open relationship or "it's complicated."