Facebook has followed Google’s lead and introduced a dual-class stock structure, the clearest sign yet that the world’s most popular social networking site is preparing for an eventual public offering.

In doing so, Mark Zuckerberg, the company’s 25-year-old chief executive, looks to be solidifying his long-term grip on the site he founded five years ago that has become the fourth most popular destination on the web.

Dual-class stock structures are controversial because they give certain shareholders much stronger voting rights than others.

“Dual-class stock is an anathema to institutional investors,” said Charles Elson, professor of corporate governance at the University of Delaware. “Ultimately, shareholders lose. If something were to go wrong, there’s absolutely nothing they can do about it.”

The use of separate classes of shares to protect the voting control of a narrow group of investors has traditionally been used in the US only in the media business. Public investors in companies such as the New York Times, Comcast and Hollinger have not been able to exert the control they would normally have, Mr Elson said.

Google adopted the practice when it went public in 2004, granting class-B shares to co-founders Larry Page and Sergey Brin, and chief executive Eric Schmidt.

Google’s class-B shares held 10 times the voting rights as its class-A shares, an arrangement mimicked by Facebook.

At the time, Mr Page warned that share structure would leave control of the company substantially with him and Mr Brin, though he added that that was necessary to fulfil the company’s long-term vision.

With Tuesday’s move, Mr Zuckerberg appears to be emulating this strategy.

Facebook confirmed the plan in a statement, saying that “existing shareholders wanted to maintain control over voting on certain issues to help ensure the company can continue to focus on the long term to build a great business”.

Mr Zuckerberg has said that he plans to take the company public eventually. But on Tuesday Facebook said an IPO was not imminent.

Facebook will convert all current shareholders to class-B stock, according to the Wall Street Journal, which first reported the plan.

Shares that carry higher voting rights usually lose that privilege when they change hands, leaving control with an even more concentrated group of investors over the long term.