Facebook has reportedly raised $500 million, $450 million from Goldman Sachs and another $50 million from Russian investment group, Digital Sky Technologies. Facebook, Twitter, LinkedIn and Zynga Game Network and also the focus of an SEC probe into private shares trading.

It’s not clear what the SEC inquiry is focused on, but DealBook and other publications have suggested that the agency may be interested in a rule that limits private companies to fewer than 500 shareholders. After a company passes 500 shareholders, it must register its shares with the SEC and publicly disclose its financial results, according to SEC rules.

Reports: Facebook raises $500 million, faces SEC inquiry [Macworld]