Here's what we know about Mark Zuckerberg: He works at Facebook (in fact, he runs the place). He studied computer science at Harvard. He speaks some Mandarin, apparently. He's from Dobbs Ferry, NY, has three sisters, identifies as a "cultural" Jew, and once roasted a goat. He is, he says, "trying to make the world a more open place."
All of this is publicly listed on the CEO's Facebook profile . The very point of Facebook, as Zuckerberg's "About me" quote reveals is openness, connectivity, sharing. Zuckerberg reportedly  "doesn't believe in privacy."
Here's what we don't know, despite our best efforts, about Zuckerberg's company: We don't know exactly how much revenue Facebook generates. We don't know how many shareholders it has. We don't know many of the things anyone can easily discover about the world's largest companies, because Facebook, despite its gargantuan stature and proselytizing about openness and sharing, has chosen time and again to avoid an initial public offering. "Don't hold your breath," Zuckerberg said  at an industry conference in November.
And now, according to a New York Times report,  Goldman Sachs has invested $450 million in Facebook in a deal that values the company at $50 billion. For Facebook, the massive infusion gives it many of the same powers as a publicly traded behemoth--without the pesky oversight of the public. Ironically, Goldman's cash and all of the heft that comes with it also comes with a heap of pressure for an IPO. The writing might as well be on Mark's "Wall."
For now, according to the Times, Goldman appears to have crafted plans to help Facebook get around the 499 investor limit on private companies by creating a "special purpose vehicle" that could technically count as one shareholder, even though it might pool investments from thousands of its high-net-worth clients. But by investing such a colossal sum, Goldman Sachs, the Wall Street firm that is quite used to winning (and has a history of friending  companies on the verge of going public), has more or less bought itself the right to take the company public, which, as Felix Salmon points out , will earn the company millions in fees.
It's about time Facebook officially came to Wall Street. Facebook has grown to such proportions that a "shadow market" of private trading in the company's stock had emerged . SecondMarket and SharesPost are two online marketplaces that have been making the deals. "We are serving a growing need," David Weir, SharesPost's chief, told the Times. "A decade ago, these companies would be public by now." The rate of this backdoor trading reached such a fever pitch of late that the Securities and Exchange Commission began to take an interest, sending information requests to the parties involved. The S.E.C. might have been investigating whether the trading has given Facebook more than 499 shareholders; if so, Facebook would be forced to disclose its financial results publicly, per S.E.C. regulations.
Lots of people are eager for the day when Facebook goes fully public, enabling the common investor to get a piece of the Facebook action. The longer it keeps its settings to private or semi-private the longer that trading and prospecting in its stocks remains a playground for the wealthy only. It's your data, and your eyeballs, that have made Facebook a $50-billion company. Someday soon (perhaps next year), you should be able to buy a small sliver of that company; and Facebook, the business, will be as open as Facebook, the website.
Earlier: The Facebook Valuation Timeline 
[Image: Flickr user Robert Scoble ]