Although Playdom chairman Rick Thompson is personally investing in other social gaming startups, the company itself has just put $5 million into Argentina-based developer MetroGames. The investment is an unusual move for a social gaming company; acquisitions have been the norm.

Although MetroGames has popped up from time to time on our weekly gainers lists, it has stayed pretty quiet — although its new city-building game, Towner, is growing fast. We currently count 8 of its games on AppData, our independent traffic analysis service for Facebook apps. The company says it currently has more than 30 total between Facebook and the web. Playdom chief executive John Pleasants will join its board.

More, from Damian Harburguer in the press release:

We are very confident MetroGames will become one of the world leaders in social gaming during this next wave of explosive growth in the industry. We already have more than 30 games online across Facebook and our own social gaming platform and many more to be released during 2010. We are pleased Playdom has recognized our potential and chosen to invest in us.

The press release says that this money is just part of its first venture round of funding, so the company may have raised more, or could be looking to do so.

Playdom is one of the largest social game developers, with strong presences on Facebook and MySpace. It raised $43 million on a $260 million pre-money valuation last fall and bought social app developer Green Patch and iPhone developer Trippert Labs; it most recently bought OffBeat Creations, earlier this month. Its latest game, Social City, is currently taking our charts by storm.