Russia's Digital Sky Technologies said it will pay $14.77 a share for Facebook common stock, boosting its stake to as much as 3.5 percent and valuing the world's largest online social network at about $6.5 billion.

While that is below the $10 billion valuation set by Digital Sky's May investment in Facebook, which was for preferred shares, investors have been valuing the social network's common stock at less than $5 billion in secondary markets in recent weeks.

Digital Sky, a Russian investment firm, bought $200 million worth of preferred shares in Facebook in May and said it would buy another $100 million worth of common shares from Facebook employees and ex-employees.

A source familiar with the matter told Reuters that Digital Sky will pay $14.77 per common share. A representative for Digital Sky confirmed the terms, and said the tender offer begins on Monday and runs through August.

Digital Sky spokeswoman Jennifer Gill would not say whether Digital Sky would impose a cap on the amount of shares that participants can sell in the offer. The firm plans to buy up to $100 million of Facebook common stock.

In a statement, Facebook CEO Mark Zuckerberg said he was pleased that the price that DST is offering is much greater than the price his company originally considered last fall in a similar program to allow employees to cash out their shares.

Facebook put that plan on hold as the financial markets tanked last year.

Facebook employees and ex-employees are eligible to participate in the offer, said Gill, but she noted that Facebook senior management, board members and holders of 5 percent or more of Facebook are not eligible due to legal reasons.

When Facebook initially attempted to create a program for employees to sell shares last year, participants were limited to selling 20 percent of their holdings, or $700,000, whichever was less, according to a former Facebook employee.

In the weeks prior to Monday's pricing, investors in secondary markets had been valuing Facebook common stock between $10 and $10.50 a share, or up to $4.7 billion, according to Adam Oliveri. a managing director of SecondMarket, which provides a marketplace for trading in private shares and other illiquid assets.

Facebook recently surpassed 200 million active users on its social network, up from 100 million users less than a year earlier.

In 2007, Microsoft Corp invested $240 million in Facebook preferred shares, snagging a 1.6 percent stake, though that deal also included other elements such as an advertising partnership. That deal had valued Facebook at $15 billion.

(Reporting by Alexei Oreskovic; Editing by Phil Berlowitz, Tiffany Wu and Bernard Orr)