Facebook.com CEO Mark Zuckerberg is shown at his office in Palo Alto, Calif. While some investors purchasing stakes in the company have valued the social networking site at more than $6.5 billion US, Facebook's own internal appraisal has pegged the company's value at $3.7 billion US. (Paul Sakuma/Associated Press)

How much is Facebook worth? A Russian investment firm appears to put it at $6.5 billion US to $10 billion US.

The investment group, Digital Sky Technologies, said Monday it would buy up to $100 million worth of Facebook shares from current and former employees. At the price the firm would pay, $14.77 US per share, the internet networking company would be worth about $6.5 billion US.

In May, Digital Sky bought $200 million US worth of preferred shares in Facebook. That valued the company at roughly $10 billion US, but those shares are worth more because they come with more rights. Facebook's open-market price would likely fall between the values implied by the preferred and common shares.

A true test would come if Facebook pursued an initial public offering, which founder and CEO Mark Zuckerberg has said he is no hurry to do.

At the very least, it seems Facebook is worth less than $15 billion US, which was implied in 2007 when Microsoft bought a 1.6 per cent stake in the company's preferred shares for $240 million US.

But based on Digital Sky's investment, it is worth more than the $3.7 billion US value that Facebook placed on itself after the Microsoft deal, according to details revealed last year in a legal settlement.

Based in London and Moscow, Digital Sky also holds a stake in Vkontakte, a Russian social network that is much more popular in that country than Facebook. If its $100-million US tender offer is fully paid out, the company will end up with a 3.5 per cent stake in Facebook.

The offer is not open to Zuckerberg and other Facebook executives.