From what I've read, it's Python and "heavily modified" Django. I recall one of their engineers saying that, if they were to do it again, they would've gone with a more lightweight Python framework because of how many heavy modifications they made.
I have read that also (multiple times actually) and the statement never made sense to me - of course in hindsight you would start with something like Flask because you ran into all of the scaling problems with Django, but there would be no way to fix these problems before they occurred - and if you did try to fix them before they would occurred, it would be premature optimization. It seems like a example of what Taleb's calls the "Narrative Fallacy"
