Flurry (they provide analytics to mobile games). They looked at more than 60,000 users who play mobile social games (which means games ranging from EA's Pogo lineup to Words With Friends to Farmville), and here's some of what they found.
Mobile social gamers average 28 years old, about six years younger than console or PC gamers, and women make up 53% of the audience. There's a lot more data presented in the study, and it's worth a look if you're developing mobile social games.
It's still tough to get found, but at least social games have a built-in marketing mechanism where players tend to rope in more players. Of course, for that to work really well you have to have lots of players... which is the classic chicken-egg problem. If you can make it to a certain size, though, your audience will start to grow nicely of its own marketing.
GigaOm has some interesting thoughts on the meaning of this to advertisers and developers. It's a different demographic than traditional games, so it's a good place for advertisers to find consumers they don't reach with other games on other platforms. Best of all, mobile social gamers tend to have a lot of money and aren't adverse at all to spending it on virtual goods.
I do love Flurry and other analytics firms. Back in the stone age of game marketing (i.e., the 1980's) we just had to guess at demographics. Now we can get huge piles of data, and the problem has shifted to one of proper analysis of the data. You have to know enough to ask the right questions, and then know what to do with the answers. It's an exciting time, though, to be a marketer.