A few examples: Ruby on rails to java (twitter). Java & c++ (google). Java (linked in). Python/Node -> Java/Go (uber). Or they start doing silly things like make a new VM (facebook).
Also, I would assume Uber probably has hiring problems in general at this point.
This might explain Facebook and PHP, but it doesn't explain the stuff mentioned in the previous comment.
I don't think threading is a relevant factor for many of those decisions. Perf and static typing, sure.
They move to static typed because their team is 100+ developers none of which knows the entire code base - and static typing helps lower the bugs possible.
Walmart also moved to node.js.
Hype was real.
