In Apple's fiscal 2011, iTunes represented 6% of its net sales, coming in at $6.3 billion.
In the most recent holiday quarter, iTunes clocked $2 billion in revenues. To put that in comparison, in that same quarter, Facebook did $1.1 billion in revenue. Even Google (GOOG) overall only did $10 billion in revenues for the quarter.
So how important might iTunes become? The answer to that is the same answer to how prevalent iOS devices will become. At last week's iPad launch event, Tim Cook remarked that more than 315 million iOS devices have now been sold. That compares to the 300 million Android devices that it has "activated." In 2011, Apple sold 172 million iOS devices. So, more than half of all iOS devices out there in the universe were sold in the last year. On the one hand, if I own multiple iOS devices myself, I get to share my apps, music and games across my devices. Therefore, the amount of iTunes revenue per iOS user shouldn't increase linearly with the number of devices sold. However, when I have more devices, I'm much likelier to learn about, participate with, and buy stuff from iTunes. Even more important for Apple, I'm also likelier to buy some of Apple's newer product offerings like iTunes Match and iCloud, just so I can keep everything straight among all of my devices.