Both DeNA and GREE publicly break down how much they are making through ads as opposed to virtual item sales in their financial reports. Virtual item sales are clearly outweighing ad revenue for both companies, but they stay mum regarding the question whose social games are actually the money makers.
The short answer is that the information isn’t public. But judging from what I have been hearing in recent months (after DeNA and GREE opened their platforms to other game makers), it seems like GREE is generating significantly more than 50% of sales with their own games (I heard numbers ranging between 60 to 80%).
The number for DeNA’s Mobage-town is much lower, with most people I have talked to speculating that it’s close to 50%.