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.

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.)

But keeping a prime position on crowded smartphone screens isn’t easy. Just ask Foursquare, which debuted with a splash in 2009 but has drifted into near irrelevance, or Yo, an app that lets you send the word “yo” to your friends, which momentarily topped the charts in 2014.

The San Francisco start-up has needed little capital so far, and the founders have turned away dozens of potential investors. However, the men, both Stanford alumni, have not been shy about seeking advice from their array of Silicon Valley connections, including senior tech executives, venture capitalists and other company founders.

As thousands of app developers have discovered, attention spans are short, especially among the college and high school students that Down to Lunch is targeting. Dozens of competitors are vying to help people organize spontaneous gatherings, including Hangster, Shortnotice, Down to Hang and a Google app called Who’s Down.

And a lot of things can go wrong on the road to becoming the next Snapchat.

I wonder why it's not as popular on Android.