I'm not sure it's a good argument to take up the biggest companies and then tally up effects of a micro improvement. You could argue for all kinds of complexity increasing changes resulting in %0.01 efficiency improvements this way.
At my company, 304s account for 3% of our CDN requests.
304 requests are so tiny that you probably end up in the order of 0.01%.
Very few entities operate at "facebook scale".
But they still account for a lot of the Web traffic.