Answer 1

https://www.sololearn.com/discuss/947465/?ref=app
https://www.sololearn.com/discuss/953091/?ref=app
https://www.sololearn.com/discuss/893697/?ref=app
https://www.sololearn.com/discuss/828671/?ref=app
https://www.sololearn.com/discuss/684221/?ref=app

Answer 2

Different parts of the Facebook platform were written in different languages that best suited the requirement. For example the Facebook AI research team would use Python. It all depends on which components you are talking about.