I'm a UX designer and Facebook baffles me. Maybe I'm a bit too old or I don't use it enough, but I find it so unintuitive - there's all kinds of options, corners and dead ends where things may or may not be hidden. And then once I've found something, getting back there again is almost impossible.
It just doesn't lend itself to a consistent mental model.
To be fair, Facebook's UX is always going to be a bit of a nightmare just due to the sheer volume of disparate features stuffed in - the 'iTunes effect' as I've just called it - so the move to focused apps for specific features is probably a good thing.
To be not so fair, I get the suspicion that the UX is left deliberately confusing in places, especially the constant rearranging of privacy settings, to get people to share more than they would otherwise.
Not sure what the back end is written in, saw a reference to Flask in a javascript comment, so maybe Python.