Node js vs ruby vs php vs python vs asp.net as backend language for social network

Answer 1

Too general, I'm afraid. A lot depends on what the setup already is. If you want to build frim scratch, try to predict the future scale and specifics, to make the best fit.
All are powerful, but some may be better suited for small-scale-quick-response projects while others for large scale businesses. Not to mention that some may have powerful tools which may be awesome, but paid.

Answer 2

Node js is javascript framework - top rated in 2017
php - senior and strong community
python - growing rapidly in 2017
asp.net, ruby - don't know much about these