Public radio is government supported, not government run. Meaning, most public radio stations get less than 5% of their annual funding from the government. The rest comes from listener donations and pledge drives, not advertisers, which is pretty much the opposite of what you're suggesting. This is also how Wikipedia works, minus the small percentage of government funding. Not sure if say, Facebook, could operate in the same way, mostly due to hardware scaling and liability issues, but it's possible.