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

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

In May, Digital Sky bought $200 million worth of preferred shares in Facebook. That valued the company at roughly $10 billion, 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 , which founder and CEO has said he is no hurry to do.

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

But based on Digital Sky's investment it is worth more than the $3.7 billion 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 tender offer is fully paid out, the company will end up with a 3.5 percent stake in Facebook.

The offer is not open to Zuckerberg and other executives.

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