They also regularly delete their Instagram photos so that their profiles never have more than a handful at a time. For comparison, I’m a medium-level Instagram user and have several hundred. They reacted to this information as if it were the smell of warm garbage.

“I have zero right now,” Lucy said.

“Yeah, ’cause I’m like, ‘Oh wait, I look stupid in this one,’ ” Leila said.

Some of Leila’s rules for Instagram include never posting more than one photo a week, avoiding photo filters (too fake) and hashtags (too desperate). She tries to find a timely occasion to post — such as National Watermelon Day — and is so concerned about having the right caption that she keeps a running list of ideas on her iPhone. Neither girl had any such rules for Facebook, because they hardly use it.

App makers fear this kind of juggling the way TV networks fear DVRs. Each time someone leaves one app for another, there is a chance that user will never come back. And since apps make money only when users are plugged in and absorbing ads, the number of monthly users is less important than how many users they get each day — and how long they stay.

Social media apps and messaging services — Wishbone included — tend to get an outsize portion of their ad revenue from a handful of mobile game makers and other app download ads. But Wishbone is making the not-terribly-crazy bet that as people spend more time with their phones and advertisers become comfortable with the medium, more brands and money will follow.

For now big advertisers remain focused on the millennial generation, who, at about 18 to 35 years old, are old enough to buy cars, homes and other big-ticket items. But an early wave is starting to think about the next group, said Erna Alfred Liousas, an analyst at Forrester Research, who said the firm had a number of financial services and media companies ask for studies on the under-17 group.

As with coffee and newspapers, the key to a successful app is to make it a daily habit. Which is why in early September, Mr. Jones of Science sat in a cinder block room staring at a computer screen full of data. He was with Benoit Vatere, head of Science’s mobile group, and Peter Pham, the company’s chief business officer, discussing the best time to send push notifications alerting Wishbone users to new polls.