Mountain View-based social gaming developer Playdom said Thursday that it has acquired Metaplace, a virtual worlds software firm based in San Diego. Financial details of the acquisition were not announced. Metaplace was backed by Marc Andreessen, Ben Horowitz, Crescendo Ventures and Charles River Ventures, and started by Raph Koster and John Donham. Koster and Donham were behind Sony Online Entertainment's Everquest II and Star Wars Galaxies titles.