DST has agreed to pay $14.77 per share, giving Facebook's common stock a value of about $6.5bn.
Mark Zuckerberg, CEO of Facebook, said in a statement: "This is recognition of Facebook's growth and progress towards making the world more open and connected
DST indicated it was planning to offer to purchase at least $100m of Facebook's common stock in May after it bought $200m in preferred stock from the social networking site, valuing it at $10bn.
The deal today carries a valuation of $6.5bn because it involves Facebook's common shares, which have fewer privileges and guarantees than preferred stock.
If enough employees take DST up on its offer, it will give the investor a further 1.54% stake in Facebook, taking its total ownership to 3.5%.
Based in London and Moscow, DST is an investor in a number of internet companies, holding significant interests in Russia and Eastern Europe, such as Mail.ru, the leading Russian-language website.
Earlier this month, board member Marc Andreessen said Facebook is set to earn in excess of £500m (£307m) this year and will likely be posting billions of dollars in revenue in five years.
Andreessen, who founded Netscape and has just launched a venture capital fund with another former Netscape executive Ben Horowitz, said it was a mistake for Facebook shareholders to be selling their stock now.
Source: Brand Republic