Nielsen has published data on the Android apps with the greatest "active reach" by age group (US market). Active reach means "percentage of Android owners who used the app within the past 30 days." After the Android Market app itself, Facebook is dominant across age categories.
After Facebook, Google occupies the next four slots with slight differences by segment. But basically it goes: GMail, Google Maps, Google Search and YouTube. In the top 100 free apps in the iTunes store, Facebook comes in at #24, Twitter at #48 and Google at #61.
In September here's what Nielsen said about overall active reach of Android apps:
Below is a chart (UK data from 12/10) that shows how dominant Facebook is in terms of time spent in aggregate minutes: