If Mark Zuckerberg hadn’t built Facebook, he probably would’ve taken an engineering job at Microsoft.

At least that’s what he told a packed audience of budding entrepreneurs and hackers at Startup School, the annual one-day conference hosted by Y Combinator at Stanford University. Zuckerberg was one of 12 prominent tech figures offering their perspectives and advice to the invite-only crowd. Speakers included both founders (Pinterest’s Ben Silbermann, Uber’s Travis Kalanick, GitHub’s Tom Preston-Werner) and funders (SV Angel’s Ron Conway, Andreesen Horowitz’s Ben Horowitz and Y Combinator’s Jessica Livingston).

Here are five valuable takeaways for entrepreneurs:

"Stupid" ideas are usually the best ideas. 

Of the many tech start-ups hatched each year, Andreesen Horowitz founding partner Ben Horowitz said that only 15 ever generate $100 million in annual revenue.

For those that do, success usually begins with a breakthrough idea.

“By definition, a breakthrough idea looks like a stupid idea,” Horowitz said. “If everybody recognized the idea as a breakthrough idea, it wouldn't be a breakthrough at all.”

One way to figure out if you’re onto something big? "Breakthrough ideas usually come from guys who look like they're hallucinating,” Horowitz added.

Let users guide your product evolution.  

Facebook may be the most popular online destination to share photos today, but the decision to allow photo sharing actually came from watching users. Zuckerberg explained that the decision came only after the company noticed how frequently users were changing profile pictures.

“We really listened to what our users wanted, both qualitatively listening to the words they say, and quantitatively looking at behavior that they take,” Zuckerberg told moderator Paul Graham.

Flexibility is underrated. 

Zuckerberg also advised entrepreneurs to make sure they have the ability to change the direction of a company.

“I have this fear of getting locked into doing things that are not the most impactful things you can do,” he said. “I think people really undervalue the option value in flexibility.”

And for founders who haven’t started a company yet, flexibility can be even more important. “Explore what you want to do before committing,” said Zuckerberg, who admitted that Facebook started out not as a company but a project. “You’re going to change what you do.”

Embrace the old school. 

Pinterest co-founder Ben Silbermann said the website's early growth was not driven by better engineering, but by better outreach. While Silbermann said only 3,000 users were registered on Pinterest three months after its launch, he knew that its core users loved the product.

“Instead of changing it, we’d find more people like me,” he said. So the company drove growth I other ways, hosting local meet-ups and starting campaigns such as “Pin It Forward,” which allowed bloggers to get more invites by inviting friends to create their own pinboards. For start-ups lacking engagement, Pinterest shows that better technology isn’t always the answer.

Give your users a reason to be a fan of your brand. 

Last week, car service Uber was forced to shut down its mobile taxi app in New York but CEO Travis Kalanick said the company is doing just fine.

“We are growing fast [at] 29% month-over-month,” Kalanick said. One reason for the company’s growth is its unique approach to marketing which focuses on giving Uber users reason to gush about the service.

For example, Kalanick explained that on Valentine’s Day, Uber gave a rose to every woman who used the mobile app. And on President’s Day, Uber users in Washington D.C. had the chance to ride the “Ubercade,” a motorcade consisting of two escalades and a town car in Washington D.C.