Speaking at the Mobile World Congress in Barcelona this week, the 30-year-old Facebook CEO revealed the main thing he looks for in a prospective employee. And the answer is simple.

"I will only hire someone to work directly for me if I would work for that person,” Mr Zuckerberg told the audience. "It's a pretty good test and I think this rule has served me well.”'

Mr Zuckerberg, who has a net worth of around $35 billion, said that he and his team looks for people whose values align with the company’s.

"Facebook is not a company for everyone in the world,” he told those gathered at world's biggest wireless phone fair during a Q&A.

One of those people who clearly passed the test is Sheryl Sandberg, the company’s chief operating officer, whom Mr Zuckerberg hand-picked for the role.

When asked by an audience member what it is like to work with Mrs Sandberg, Mr Zuckerberg said he considered her a mentor and someone who has been instrumental in building Facebook into a business and "healthy organisation.”

Facebook’s staff is relatively small compared to other tech giants in Silicon Valley, such as Google, which has almost 55,000 employees. Mr Zuckerberg said that was crucial part of the social media company’s success.

"The most important thing is to keep your team as small as possible," he went on. "[Facebook] serves more than a billion people around the world but our team has fewer than 10,000 people. It's only possible because of modern technology. Big companies get bloated."

He said the most important thing is to "just have faith in yourself and trust yourself.

"When you’re young you hear that you don’t have experience to do things, that there are people that have more experience than you. I started Facebook when I was 19

"Don’t discount yourself, no matter what you’re doing,” he added. "Everyone has a unique perspective that they can bring to the world.”

He was asked if the world’s biggest social network had ambitions outside of social media and the Internet, but said Facebook is "pretty focused.”

Facebook’s goal of connecting people globally is "pretty broad,” he said. That includes enabling people to share their own photos and videos and message each other as well as giving businesses a platform.

Facebook is also trying to get more people in developing countries online through its Internet.org push and developing its virtual-reality technology.

(telegraph.co.uk)