Forgive a bit of self-advertising, but I also thought some of you genuinely might be interested in this.
It also (perhaps more importantly) has its own social network (it's the child of the Plus network in the US and the mobage network in Japan) with already has millions of users. Included with that are many Steam-like features, like achievements, buddy lists, etc. It also has a virtual store, so all monetary stuff is taken care of for you, similar to in-app purchases on iTunes (on iOS it does go through the iTunes store).
It's also free to use. I ported most of my game Sinuosity to it as a test in about 1 week, so it's pretty easy to use.https://developer.mobage.com/
How it works