Playdom, a fast-growing social game company, is investing $5 million in Argentina’s MetroGames as part of an expansion in social games.

The deal is somewhat unusual one for San Francisco-based Playdom, which is the biggest social game publisher on MySpace and has acquired three other social game developers recently. Playdom raised $43 million in venture funding last year to fund its expansion; now it is taking $5 million of that money and putting it into another company.

MetroGames is based in Buenos Aires and is working on a pipeline of games, including those for its Metrogames.com social game platform. MetroGames has more than 30 games on Facebook and plans to release many more in 2010, said Damian Harburguer, chief executive of the company.

John Pleasants, chief executive of Playdom, will join the board of MetroGames.