There hasn't been much talk about it, but I think advertisements will be one of the biggest sources of income for Glu's games. You have to keep in mind that smartphones continue to be rolled out and companies will pay bigger and bigger money to advertise on these games in the future.
All Glu has to do is keep making good games and gamers will go to them. Then the advertising dollars will really start to flow in. The CEO recently said himself in an interview that adverstising was going to be a big source of revenue.