Nobody’s getting “the last .1% that can ever be gotten”, so it’s a moot point. Google and Facebook are still growing. There’s always room for more improvements.
They have ridiculous numbers of servers. Rather than fiddling to reduce server count, they're improving their ability to scale out. It's cheaper and it's repeatable. Doing crazy things like building custom hardware and power distribution buses (Facebook) to reduce heat in the data centers.
There are services where latency is important, they launch multiple requests to different replicas but to optimize this when one of the replicas is serving the response it sends a request cancel to the other one.
Horizontal scaling and vertical scaling is both very important.
Both G and FB pours many engineering hours into maximizing per server "ROI". Just think about how much they work on scheduling work/tasks so they can do more with the same number of servers.