Facebook boss Mark Zuckerberg has revealed he follows one simple rule when hiring staff.

The billionaire entrepreneur, was addressing an audience at the Mobile World Congress - the world's biggest wireless phone fair in Barcelona – when he was asked during a Q and A what is the key attribute he looks for in a potential employee.

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

"Facebook is not a company for everyone in the world," he added.

Facebook is regarded as one of the toughest technology companies to get hired by because it has a relatively small staff of 10,000 compared with other Silicon Valley firms.

Mr Zuckerberg said: "The most important thing is to keep your team as small as possible. 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."

Zuckerberg, who is worth around $35bn (£22.9bn), hand-picked his chief operating officer Sheryl Sandberg, He told the audience he considered her a mentor who had helped him create a "healthy organisation".

Earlier this week, Zuckerberg came under fire following complaints that his social network was getting a free ride out of telecom operators who host its service on smartphones.

Executives have complained that some Internet services' smartphone applications generate revenue while it is the telephone companies that are forced to invest in the networks that host them.

Zuckerberg later defended himself saying: "We have these services that people love and that are drivers of data usage... and we want to work this out so that way it's a profitable model for our partners."