Social games are becoming a mass-market medium. A clear sign of that today is Zynga’s announcement that Martha Stewart will make a cameo appearance in the social game CastleVille.

This is the first celebrity integration for the game, which has 26 million monthly active users. Players can interact with Martha’s avatar and get special in-game rewards. Martha has her own neighboring kingdom, designed after her home in Bedford, Conn. The property has new animals such as a Friesian horse, Araucana chickens, and black sheep. It also has new flowers, black orchids and white tulips, and the largest building ever introduced in CastleVille (her home).

Stewart will host an Easter egg hunt in her kingdom and celebrate with a party. Players can create a special gazebo and work on crafts such as egg decorating. I’m not exactly sure where, but somebody is making some money off this. Stewart certainly hits the older female demographic of Zynga’s games on Facebook.