It is unclear whether or not Facebook has responded to the offer from Digital Sky Technologies, which is controlled by Russian internet entrepreneur Yuri Milner.
DST has proposed to pay $200m for Facebook's preferred shares, and in a second part of its offer to buy between $100m and $150m of Facebook's common shares.
Facebook chief executive Mark Zuckerberg has said the company is considering raising more money, but does not have to.
It has not accepted any significant investment since selling a 1.6% stake to Microsoft at the end of 2007. The deal valued Facebook at $15bn.
DST owns stakes in several Russian internet businesses including the country's largest website, Mail.ru.