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.
This is one of the things that gets me. If you get the last .1% that can ever be gotten, what do you do for an encore?
When your userbase grows another 5% or 10% or 20%, it won't be enough. You'd be better off trying to figure out ways to give your users something they want that doesn't require most exotic thing that can be procured. It's expensive to begin with, and it's the end of the road. You don't want to be at the end of a road trying to figure out what to do next.