Chitika's newest data regarding ad impressions from last quarter shows Google's Android OS pulling ahead of Apple's iOS. Android's marketshare rose to 51.6% in December, while iOS commanded 46.5% of ad impressions on Chitika’s network. Of course, it is important to note that a decent part of these percentages have extra "padding" in the Android side of things. This is because Android has more free apps that are ad-supported than you see on the iOS side of things.

Still, these numbers have been steadily increasing over the past year, so they are, at least somewhat, indicative of the upward momentum of Android.

Source: PhoneArena