Open source projects are not run professionally. I don't recommend getting involved with them as a career move.

How about joining the Microsoft campus instead? Learn about the full software engineering cycle. Now, that would be something!

Linux is pretty terrible for performance multi-threading, that's a fact. It features unreliable file IO too, but I digress..

In the case of Facebook, it's true that it's not the OS fault since Mutexes are always slow anyway.

There are lockless libraries that lock the CPU(s) for one cycle so that the program doesn't need to lock a mutex to increment a counter, for example. Thousands of times faster...

But these wouldn't have helped there. Like you said, it just seems like a design problem in the software. Still 140M users is very impressive.