Lawyers representing Facebook sent a letter last week to at least one of the special markets where private company shares are traded informing them of the move, the source said. The letter, from the law firm Fenwick & West, did not provide a reason for temporarily halting private transactions of Facebook shares.

The suspension, which runs from Wednesday to Friday, comes as anticipation is building for Facebook to sell shares to the public later this year.

The fact that Facebook will not honor secondary market trades in its shares for three days does not necessarily mean the company is getting closer to filing a prospectus for an offering.

Facebook officials declined to comment. News of the suspension in honoring Facebook trades was first reported by Bloomberg News on Tuesday.

Shares of Facebook, the world's largest Internet social networking company with more than 800 million users, recently traded at $34 a share on SharesPost, according to information posted on the website. That gives Facebook an implied valuation of $80 billion, according to SharesPost.

(Reporting By Alexei Oreskovic; Editing by Matt Driskill)

• A Permanent Economic Emergency