Betting bigger on social casino games, RealNetworks has agreed to buy Slingo Inc. for $15.6 million. RealNetworks has a big GameHouse casual games division that is expanding into social casino games in a big way. By purchasing Slingo, RealNetworks can capture one of the most popular brands in the business and use it to make more social casino games. It’s not clear what this means for Zynga, which has a partnership with Slingo on a Facebook version of the popular Slingo game. Slingo also operates its own games on Slingo.com. The Slingo-Zynga game on Facebook has 4.5 million monthly players….