SAN FRANCISCO — Nikil Viswanathan and Joseph Lau have built the hottest new social app in America. Now the young men have to keep it from getting crushed by an anonymous slander campaign, overwhelmed servers and their urge to personally respond to thousands of messages from users.

The app, called Down to Lunch, is shockingly old-fashioned: It’s all about meeting up with your friends in person. You send a message to some or all of your buddies saying that you have free time and are looking for company for a meal, a gym workout, even a church service. Whoever is interested responds and you arrange to meet.

“We’re trying to make it feel like you live with your friends again in your freshman dorm,” said Mr. Viswanathan, a Texas native who graduated from Stanford University in 2012 with a master’s in computer science.

The concept is so simple that the first version was built in a day last spring. By last week, Down to Lunch, also known as DTL, was the No. 1 free social networking app for the iPhone and the No. 2 free iPhone app over all. (It doesn’t rank quite as high on Android.)