Facebook has been valued at $6.5bn by Russian internet investment group Digital Sky Technologies (DST).
DST, which invested $200m in preferred stock in May, has confirmed that it will purchase up to $100m in common stock from existing shareholders.
The full common stock investment will buy an additional 1.54% share in Facebook to bring DST's total stake to 3.5%.
DST has joined a select group of stakeholders in Facebook, but it is not one of the largest, according to the Financial Times.
Early investors, including Microsoft, are thought to hold much larger stakes in the social networking business.
The common stock purchase was part of DST's initial investment in May and is seen as a way to retain employees until Facebook makes its initial public offering.
The common stock investment will "facilitate liquidity for current and former employees' vested shares in the company", Facebook said.
The partnership is thought to be strategic for Facebook's growth in Russia because of DST's government connections and experience in making money through social networking sites.