Mobile game ad network Chartboost has been drawing online buzz thanks to a big investment round. Silicon Valley venture capital group Sequoia Capital just led a second round of funding totaling $19 million for Chartboost, a large sum for a company whose first round was for $2 million back in 2011. Chartboost boasts that its cross-promotional mobile ad network has grown to reach over 300 million monthly active devices across 12,000 games. It claims to differentiate itself from other networks by offering a free SDK for developers to cross-promote apps at no charge. Optionally, developers can also sell that same ad space for targeted apps on Chartboost's network on a revenue share basis. More information on the news at Chartboost's blog.