Early Facebook employee Noah Kagan says Mark Zuckerberg always asked one question when his team had ideas:

In a 2014 interview with Ramit Sethi, author of the bestselling "I Will Teach You to Be Rich," Kagan shared the No. 1 insight he gained from his short-lived Facebook career: "When you are running your business, have one goal. That's it."

Kagan explained that he would continually approach Mark Zuckerberg with specific ideas that would help Facebook make money. But Zuckerberg would always ask the same question: "Does it help us grow?" If not, he would tell Kagan he wasn't interested.

Zuckerberg's clear focus on growth "helped us clarify every decision," Kagan said.

Now that he's in charge of his own company, Kagan makes sure to have one key goal per year. This year, his goal with the marketing tool SumoMe is to get to 1 billion users.

Kagan explained that he's broken down that goal on a monthly and daily basis to make it more manageable. For example, if this week they need to get to 600,000 users and he sees that they're only at 500,000, he says, "What do I need to do this week to go from 500 to 600? What are the three things I can do this week?"

Sethi added that Kagan helped him successfully market "I Will Teach You to Be Rich" by encouraging him to focus on just one goal. Apparently, Sethi initially had 37 pages of marketing ideas, but when he approached Kagan for advice, Kagan asked him what his one goal was.

Sethi realized it was making the New York Times bestseller list, and Kagan helped him cut his plan to just five pages. The book went on to become a New York Times bestseller.

"That was all because of your relentless focus," Sethi told Kagan, "and unwillingness to accept multiple goals."

The full interview is available to premium users on Ramit's Brain Trust.