If Facebook is the king of social networking, then Zynga is the king of social gaming and, according to a new report, would be traded at a $5 billion market cap if it went public this week.

The report, authored by former Goldman Sachs and Merrill Lynch equity analyst Lou Kerner, Eli Halliwell of Sanford Bernstein, and Gamers Media CEO Jay Gould, says that Zynga, with over one billion monthly active users, accounts for over half of the users playing games by the top ten biggest game developers on Facebook. EA, which acquired Playfish for $400 million in early November, is the second most popular social gaming company with 11% of the market.

Zynga, which many have assumed to be profitable for some time now, could be pulling in $500 million dollars in revenue this year. Not only that, but the report estimates that by 2012, Zynga might be generating over a billion dollars in revenue.

If Zynga were public today, it would be selling at $15.75/share.

To get these estimates, the report's authors turned to similar examples in China, where online gaming has been a strong force for a while. The four public Chinese companies specifically focused on online gaming see aggregate revenues upwards of $3.3 billion.

As always, these valuation estimates must be taken with a grain of salt. Just last November, Bloomberg reported that Zynga would be valued at $1 billion. No matter what the exact number would be, it's clear that Zynga is on its way.

Maker of six of the seven top games on Facebook (like FarmVille and Mafia Wars), Zynga is constantly expanding and enhancing its game collection. A couple weeks ago, the company announced that it was bringing game cards with pre-paid virtual currency to retailers across the country. And just yesterday, Zynga released a new feature for Mafia Wars that lets users continue playing over SMS.