Two weeks ago, Serious Business, Inc., makers of popular Facebook social game Friends For Sale, announced that it had raised a $4 million Series A venture round from Lightspeed Venture Partners at an undisclosed “healthy double-digit” valuation. The investment was disclosed by Lightspeed’s Jeremy Liew, who is becoming a board member, and Siqi Chen, co-founder of Serious Business, at last week’s Games 2.0 panel at Web 2.0 Expo.

Serious Business and Liew claim that their idea of baking virality into the game mechanic – instead of layering games on top of social networks and using them for distribution – is the special sauce that will allow them to build a business in social games.

Read the full interview with Serious Business co-founders Siqi Chen and Alex Le at Inside Facebook