http://www.treysmithblog.com/the-fall-of-angry-birds/
Good quote:
Right now, 18 of the top 25 grossing of all apps are Free To Play Games (72%). Also, it should be noted that 22 of the 25 top grossing apps are in the games category (88%), confirming the fact you need to be into games if you want to have the biggest potential payout. The reason for this is people have a stronger emotional attachment to games than any other type of app, therefore they are more likely to spend money.