[wmf]

It's a fad (which is fading in favor of Go). A few comments:

Backend programming would require intimate knowledge of threads, mutexes, async...

Not if they use Node.js. One reason to use it is that it doesn't allow you to use threads or to even decide whether to use threads or not.

Surely it works great with MongoDB...

And that's what people use.

Is Node.js performance really that good...

People who choose programming languages based on hype are never going to get good performance anyway, so it doesn't matter.