Social networking giant Facebook has reportedly halted trading of its shares on the secondary market, in preparation for its Initial Public Offering (IPO), which is expected in May.

Facebook filed for an IPO in early February, in an offering that could raise as much as $10 billion and value the company at $100 billion. The social network, which has 845 million users globally, made $1 billion in 2011 based on revenues of $3.7 billion (£2.3bn).

A source told Reuters that the May time frame for the IPO is dependent on the Securities and Exchange Commission (SEC) declaring the company's prospectus effective. Facebook has reportedly amended its prospectus three times since filing paperwork to go public in February, and expects to file at least one more amendment before the IPO.

There has been high demand for equity in Facebook over the years, and the number of shares being bought on the secondary market has increased since the company filed to go public. However, some investors are concerned about Facebook's dual-class share structure, which will give CEO Mark Zuckerberg control of 56.9 percent of the company's post-IPO voting shares.

Zuckerberg founded the social network from his Harvard dorm room in 2004. The site had 483 million daily active users on average in December 2011, compared to 327 million in December 2010, the company told the SEC.

“I've developed a deep appreciation for how building a strong company with a strong economic engine and strong growth can be the best way to align many people to solve important problems,” said Zuckerberg, in a letter enclosed in the SEC filing. “Simply put: we don't build services to make money; we make money to build better services.”

Zuckerberg's blasé attitude to money was made apparent when he skipped the company’s first major briefing for analysts and bankers last week, raising the ire of investors.

“He wants investors to put their money behind him, with the confidence in him personally, as the person who built this company and who's going to lead it and control it. He should be accountable to those people who are investing,” said Carin Zelenko, the director of the capital strategies department for the International Brotherhood of Teamsters.