Backend Language(s)
I guess, that they might think about using Go for their front-end facing backend, since it is highly concurrent and better suited for the job than other languages. They might use Python for other tasks, like data mining, data analysis and artificial intelligence, though. Some of those calculations will definitely be offloaded to GPGPUs, which adds in OpenCL or Cuda.
Client-Server Communication
Concerning the polling, I they'd probably go for sockets from the start, which allows them real-time notifications, updates and chat while having close to zero overhead (no HTTP request with headers, TLS handshake, etc.), meaning a better performance and less CPU-costs for handling said overhead.
Front-End
Facebook's philosophy is "Move Fast, Break Things", so I guess even if all of the tech which exists today (React included) would still be available, they would do their own things. I mean, it's a company which literally welcomes you with the word "HACK". Headquarters is located in "Hacker Way". What else can you expect? But it would be quite close to React, because that's what they need. I guess they would do backend-rendering (taken from cache) for the initial page, then offloaded rendering to the client (client capacity is free, after all)
Image/Video Processing
If I had to create something like that today, I would probably go for C or C++ with OpenCV, because of the performance per resource usage and ease of development. Since we are talking about FaceBook, they might, however, do their own thing, again, which is more tailored to their specific needs. Again, GPGPU here, so OpenCL or Cuda will also be used.