From Venture Beat – Real-money gambling and FarmVille just don’t mix. Zynga chief operations officer David Ko said in an analyst conference call today that the company decided not to invest in U.S. real-money online gambling because it needs to stay focused on social gaming and its core casual audience. The San Francisco company slightly beat Wall Street expectations, but it continues to bleed users. This, and the decision to withdraw its plans to bid for real-money online gambling licenses in the U.S., led to a tumble in the company’s stock price after-hours trading. The stock is now down about…