Better (two days) late than never.

Social gaming giant Zynga today filed for a $1 billion initial public offering. Morgan Stanley (MS) and Goldman Sachs (GS) are listed as lead underwriters. Others bankers include BoA Merrill Lynch, Barclays Capital, Goldman Sachs and Allen & Co. The company has not yet selected an exchange or ticker symbol (Zillow has dibs on “Z”).

Zynga reports $90 million of net income in 2010 on around $597 million in revenue. The revenue figures are a staggering $235 million for the first quarter of 2011, but the profit margin dropped from around 15% in 2010 to just around 5% last quarter.

Nearly 95% of the company’s revenue comes from selling virtual goods, which has increased 127% between Q1 2010 and Q1 2011. Advertising revenue increased 321% over the same period but, again, makes up a very small part of the whole.

Perhaps the most impressive figure is cash on hand. Zynga reports it has nearly $1 billion in the bank as of March 31, which is a couple hundred million more than it had at year-end 2010.

Zynga founder and CEO Mark Pincus is the company’s top shareholder, with a 16% position. He makes $300,000 per year in salary, while his number two, Owen Van Natta, makes $200,000. Pincus also leads the prospectus off with a personal letter, kind of like what Groupon CEO Andrew Mason did in that company’s S-1 last month. The only real difference is that Pincus goes more for sincerity than snark:

At Zynga, we feel a personal connection to our games through our friends and family. I love that my brother in-law, who has five kids and no free time, religiously plays our game Words with Friends…. My kids decided a few months ago that peek-a-boo was their favorite game. While it’s unlikely we can improve upon this classic, I look forward to playing Zynga games with them very soon. When they enter high school there’s no doubt that they’ll search on Google, they’ll share with their friends on Facebook and they’ll probably do a lot of shopping on Amazon. And I’m planning for Zynga to be there when they want to play.

Other significant shareholders include Kleiner Perkins Caufield & Byers (11%), Institutional Venture Partners (6.1%), Foundry Group (6.1%), Avalon Ventures (6.1%), DST Group (5.8%) and Union Square Ventures (5.5%). Some of those firms already have sold shares back to the company via secondary transactions. Union Square, for example, had over $45 million worth of shares repurchased. Foundry Group and Avalon Ventures sold back around $22 million and $21 million, respectively. For his part, Pincus liquidated $109 million worth of securities this past March.

Reid Hoffman, the LinkedIn (LNKD) founder who first seeded Zynga and sits on its board, appears to have around a 0.6% position.

The company says that most of the IPO proceeds will be used for general corporate purposes, rather than cashing out existing shareholders.

It reports more than 2,200 employees — approximately 64% of whom have been hired within the past year.

As we wrote earlier this week, Zynga could be an interesting market test for Facebook. Not for the social network’s own IPO prospects, but for how the public markets will view Facebook as a legitimate platform for other companies to build businesses on top of. Kind of like how the Microsoft or Apple operating systems are viewed today for software makers.

For its part, Zynga acknowledges the importance of Facebook to its business model. Its top risk factor is: “If we are unable to maintain a good relationship with Facebook, our business will suffer.” Moreover, Zynga acknowledges that it generates “substantially all of our revenue and players through the Facebook platform.”

The company does not say anything about timing, but it’s a safe bet that Zynga doesn’t plan to actually go public until sometime after Labor Day. At that point, don’t be surprised if the offering is actually much larger than $1 billion. Initial reports were that it was seeking to raise between $1.5 billion and $2 billion, which makes me think the current cover price is a conservative placeholder.

Finally, here is a word cloud from the S-1 (created by using Wordle):