See the Ning Blog for the original post from Marc Andreessen:I’d like to update everyone on a change happening in our company today. My fellow co-founder and Ning CEO Gina Bianchini has decided to step down after five and a half years of hard and terrific work. I support her wholeheartedly in her decision. Gina is handing the CEO duties over to Jason Rosenthal, who has been our Chief Operating O…