It's not just the social stuff - from my understanding it's running a modified Hyper-V to capture the core console OS. Presumably this segregates off the ability to do background downloads, implement the encryption for HDMI, and so on and so forth from the XBox gaming portion.
If you believe the rumours Dave Cutler worked on the hypervisor of the Xbox.
I don't know what the breakdown is (what number of cores goers to OS,number of cores for the games), the the Xbox One has a rather low performance 8-core AMD unit; the cores are provisioned amongst the OS and games as needed.
Since the CPU is low performance, the OS could easily eat 50% of its total performance, leaving scant little for games.
Isn't that 30% made to take care of the social features, like "sharing a clip of the last frag I just achieved" in a FPS or something like that ? If you want to have these advanced features, you need a number of tasks to run in the background and I would not be surprised by the 30% CPU time.