I used Lush for my homework as part of the Machine Learning course at NYU (taught by LeCun). It was pretty mature, mostly bug-free for general use. However, at that time, I never used a lisp-y language before that, and found it quite difficult to do my homeworks. It had an interesting mode where if you annotate your lush source-code with type-hints, all your Lush transpiles in to C, which you can compile with your favorite C compiler. It was eventually abandoned in usage because Torch (Lua based scientific-computing platform) was adopted by the lab.

AFAIK, Lush is not used inside Facebook labs.