Facebook’s value has dropped $3.5 billion since May.

Russian investment house Digital Sky Technologies, which put $200 million in the social networking outfit two months ago in exchange for preferred shares, now wants to buy $100 million worth of common stock from current and former employees.

It’s offering $14.77 a share.

That price values Facebook at $6.5 billion. In May the valuation was $10 billion.

But Reuters says Facebook’s common stock has been valued lately at around $4.7 billion by secondary markets that deal in private shares and other illiquid assets.

The price Digital Sky is offering is more than Facebook had in mind last year when it considered setting up a program so employees could cash out. The idea evaporated with the recession.

The two investments together would give Digital Sky a 3.5% piece of Facebook, which has yet to figure out how to monetize its 200 million active users.

Two years ago when Microsoft paid $240 million for a 1.6% stake in Facebook, the company was valued at $15 billion.

Digital Sky, whose other holdings equate to 70% of all hits on the Russian-speaking Internet, according to Wikipedia, is owned by Russian billionaire Alisher Usmanov and Goldman Sachs, among other interests.