Zynga has been getting the most attention among social gaming companies lately, for a variety of reasons, but rival Playdom has been busy with some news of its own — although not exactly official news. Its “annual revenue is upwards of $50 million,” a reliable source within the company tells us. This number is higher than any previous estimates we’ve heard.

So how is Playdom making money? Surprise! Through virtual goods — selling items like weapons in its Mobsters mafia-style role-playing game and attire in its virtual world app Sorority Life. These are games that became hits on MySpace, and have more recently been finding users on Facebook and other platforms. We don’t have a revenue breakdown between users who pay for virtual currency with real money versus those who earn currency through advertising offers; the company uses both methods.

In terms of users, Playdom says it has 28 million monthly actives. How do these numbers break down? It’s not entirely clear. The company continues to dominate the MySpace charts, with three games in the top five, according to our latest analysis. MySpace only provides total users, not monthly active users (the metric Facebook uses). But still, we can see that Mobsters on MySpace, for example, has the most total users, with nearly 14 million total as of this past month. It is, by this measure, the number one game on MySpace.

Meanwhile, over on Facebook, we can see that the company has been gaining millions of users over the last few months. It now reaches nearly 12 million monthly active users across its six applications on the platform, according to our AppData service. This number is not deduplicated however, as a large fraction of these users play more than one game, and we can’t tell who these users are — and the same goes for MySpace. Also, Playdom has been busy expanding its product line, including Mobsters, onto the iPhone.

So anyway, these 28 million self-reported monthly actives are adding up to $50 million in annualized revenue. The highest estimate we’ve previously heard about the company was this spring, when one source told us the company was making $10 million in revenue a quarter. Another source said that number was way too high — if nothing else, it no longer is. We also heard at the time that Playdom was looking to raise money, but to our knowledge it hasn’t.

To be certain, many of Playdom’s games are closely patterned after others in the same genre — see the long history of social gaming companies copying and suing each other for more on that. Still, whether through cross-promotion, advertising, and whatever other means of growth it may have discovered, it has managed to be one of the few clear market leaders.

Playdom is also aggressively looking for more ways to bring in more revenue. Yesterday, for example, it announced a deal with paid card provider InComm, whereby users can buy cards in retail stores with cash, then use codes on those cards to access equal amounts of virtual currencies within Playdom games. These cards are aimed at teens who don’t have access to credit card or mobile-account payments — 25 percent of Playdom’s users are teens.

Meanwhile, like many other gaming companies, Mountain View, Calif.-based Playdom is expanding. It has grown from 60 to more than 110 employees in the last few months, opening an office in San Francisco’s tech-heavy SOMA district, and still hiring. And in June, it picked up some gaming industry veterans, including former EA chief operating officer John Pleasants as its new CEO.

So, while Zynga has announced that it has 129 million monthly active users across the same platforms, and while we hear that company could make as much as $200 million this year, Playdom is proving itself to be a serious rival in the still-young world of social gaming.