In an email, Mr. Zuckerberg said Instagram’s founders had “created something special, and the team has taken that and helped build it into something that people around the world love.” But, he added, “we still have a lot to do to make the experience even better and make sure we’re living up to what people expect from us.”

This delicate balance — keeping both Facebook and Instagram happy and facing animosity in both camps — reminded Mr. Mosseri of his father, an Israeli-American psychotherapist who speaks Hebrew with an American accent and English with an Israeli accent.

“I feel like I speak two languages and neither is perfect,” Mr. Mosseri said. “It’s like, either place you go you get, ‘Where are you from?’”

Jogs With Zuck

Mr. Mosseri’s story began like those of many tech executives: in college. He was a freshman at N.Y.U. when he started designing websites, mostly to help pay the rent on the windowless room in the one-bathroom apartment that he shared with five roommates. He started a small web-design firm with a partner, Sidney Blank, who described it as “two guys with a couple employees futzing around.”

Mr. Mosseri’s firm got a couple of commissions from Brown University and the Architectural League of New York (his mother is an architect), including one to create an interactive rendering of what a redesigned World Trade Center might look like. In 2005, he opened a West Coast office, following a couple of friends to pursue start-up riches in San Francisco.

There, he created Boombox, a music-sharing app. Before he received a cease-and-desist order from the Recording Industry Association of America, the app caught Facebook’s eye. Mr. Mosseri’s wife, Monica, was working at Facebook in operations; Mr. Mosseri had applied there several times but never got an interview. Now, as the company eyed music sharing, he had an in.

In 2008, Mr. Mosseri joined Facebook’s design team, committing so completely that he’d sometimes crash on a sofa in Silicon Valley with other early Facebook executives rather than go home. Thinking like a designer but coding like an engineer, he embodied the work-hard-play-hard ethos that Facebook looked for in its employees at the time, said Soleio Cuervo, a former product designer at Facebook.